You know, Its difficult to say without being in the car. But in my case, there really wasn't any difficulty at all telling which bearing was bad, front or back. It was just real clear. Well, let me back up and tell the story a minute...
When I bought the truck used, it sounded like a 4wd pickup with super swampers on it. It just had a loud tire noise in general. It was loud enough that passengers commented on how much tire noise the truck made. I didn't think much of it, except cheap tires, at first. But then, as murphys law would dictate, the first time I went out of town, the front bearing suddenly went from bad to worse. It sounded so bad, I had the truck towed to the nearest Firestone and they installed a new front bearing ($387!).
When I drove the truck home, I noticed that my 4wd tire noise was suddenly limited to just the rear tires. :-( Now I knew that I had a rear wheel bearing to fix as well. I was not able to detect the problem earlier because both a front and rear bearing were bad, causing the noise to be all around me.
After getting hosed bigtime by Firestone on a new bearing in the front for 387, when I could have bought a name brand front hub assembly on Ebay for around 100 and just bolted it in, I decided to fix the rear one myself. As it turned out, that was a dumb thought. The rears took a LOT more work to replace than the front. By the way, thanks Firestone, for nicking a hole in my front axle CV boot with your impact gun, causing grease to sling out of my front cv boot all over my truck. To be fair, they are going to fix that I guess. Note to all: Be careful breaking those 3 bolts loose on the front bearing. They are on the back side of the knuckle, and if you try to use something bulky like an impact gun, you could easily rip the boot like they did.
On the back of the truck, instead of having a hub assembly that just bolts in with 3 bolts, they press the hub into the bearing, and press the bearing into the knuckle. Furthermore, there are miniature brake shoes (parking brake shoes) inside your rear rotors. I decided I would do this job myself, no matter what it took, and despite having a huge tool box already, I found myself ordering lots of new tools. Just to give you some idea of the pitfalls you will encounter doing the rear hubs, here's the highlights:
First, you'll need a 35mm socket to get the axle nut off. Many online writeups say 36, but a 36 feels loose on there, and a 35 is an exact fit... at least with my SK axle nut sockets. I bought the set for about a hundred bucks from Amazon.
You'll need a 1/2" breaker bar too, since they are torqued on at something like 211 ft lbs, if I recall.
Then you pull off the rear caliper and bracket, and suspend the caliper out of the way. I bought OTC S hooks, and hooked it into a little 1/2" hole on the front side of the wheelwell.
Next, pull off the rotor. Now if things work as they are supposed to, that might be easy. If it's tough coming off, its possible the rear brake shoes inside your rotor are binding. From what I hear, many people break the rear brake shoes. If you want to loosen the rear brake shoe tension first, you'll have to knock out a 3/4" round knockout plug in the bottom rear of the brake backing plate, and turn the adjuster so there's less tension. The knockout plug wasn't apparent to me, until I was reassembling the brakes. But it is there. Just look for a 3/4" round indention near the bottom rear of the brake backing plate, and knock it in with a punch.
When I pulled my rotors off, they came off without a fight. And for good reason. The brake linings were non-existant. All that was left of mine was about 1/3 of 1 lining. The rest was disintegrated I guess. This might have been partly due to my truck being a northern truck, and having some rust inside there.
Once the rotors are off, you have to remove the rear brake shoes. Getting the top spring off is much easier than getting it back on! I'm still cussing on that one. I didn't have any brake spring tools, so I tried to just slide the spring back into it's hole with a screwdriver. Trust me, buy some sort of brake spring tool. There's a top spring, a bottom spring, and an adjuster on the bottom. Then there's 2 clips with a T stud going thru them on the bottom of the shoes, that hold the shoes on. They come off easy. Putting them back on requires cussing. And 2 people.
Then you'll need a FRONT hub puller like this one:
http://www.tooltopia.com/index.asp?PageAction=VIEWPROD&ProdID=7750
I bought an OTC slide hammer kit, which included that puller as well as a rear axle puller, and slide hammer and misc. attachments.
Next, you push the axle in thru the knuckle using that front hub puller. Then you remove the tie rod and upper ball joint retaining nuts and bolts. The stud on each of these that goes into the knuckle has a groove in it, in which the retaining bolt partly goes thru. In other words, you have to remove the bolt in order to allow the stud to come out. And it doesn't come out easy. I suggest pulling the nut and bolt out of both the rod end and the ball joing, and spraying down in the hole with PB Blaster, and letting it sit overnite. Then smack around on the knuckle, and on the side of the upper control arm with a baby sledge the next day. They should pop out pretty easy after being soaked. If not, keep smacking and soaking with oil. It's the vibration of the smacking that will crack the stud free. This was hard at first, but not bad at all after I sprayed it with PB Blaster. Soaking it overnite would have made that much easier for me.
Once those 2 are loose, then remove the lower control arm bolt, ABS sensor, and the knuckle should come right off. Now, if you are smarter than me, you'll take the old knuckle/hub assembly and your new bearing or hub assembly down to a 4WD shop or a machine shop and get them to press out the old bearing and press in the new bearing. I was dumb enough to try to do this myself.
The bearing would not press out, which is par for the course. I was able to knock the hub out of the old bearing easily with a socket and a baby sledge. But the outer race stayed stuck in the knuckle. I had to take a 3" cutoff wheel in a drill, and cut the bearing race practically in half, once on each side of the race. Those races are incredibly hardened. After cutting the race nearly in half in 2 opposite places, I was then able to press the bearing out. Once finally removed (the hardest part of this whole job), then I took a sandpaper/scotchbrite flap wheel and polished up the ID of the knuckle to remove ridges and crud from the old bearing. Once shiny and smooth, I put some WD40 on the new race and knuckle, and pressed in the new bearing. That's tricky too, because you first have to press the new bearing in the knuckle. I used the old race to do this! Then you have to press the new hub into the new bearing. But you HAVE to support the ID of the bearing on the back side, while you press the hub in from the front side. Failure to do this will press the new bearings right out of the new race. I couldn't find socket big enough to support the bearing from the back side (it would have taken approximately a 38 - 42 mm socket), so I went to Lowes and bought the biggest ball valve they've got, from the plumbing department. That cost me 42 bucks, which I returned later on... yeah yeah, I know. Anyway, it was the only thing big enough and round and metal that I could find to support the ID of the bearing during the pressing.
That did the trick. Next, I installed a new snap ring. The tapered side of the snap ring faces UP towards you. By the way, I had to take a small screwdriver and clean out the snap ring and groove when I disassembled it. Mine was crudded in there, and wouldn't budge until I scraped out all the salt and dirt that was embedded around it. Then I used a fairly large, stout snap ring pliers to get it out.
While it's this far apart, decide if you need a new rear axle seal, where the CV axle goes into the rear end. I needed one on one side, so I bought both. Ford redesigned this seal, which costs about 25 each. It takes the 2 piece factory seal, and replaces it with a 1 piece seal. You remove the other half from the CV axle, then polish up the axle with some fine sandpaper. To get the axle out, I tried buying the slide hammer, an extension, and a CV fork. But the fork wasn't big enough, so i resorted to a 1x1 by 10" long block of wood, and a baby sledge. Hit it as straight forward towards the knuckle as you can, from the front side of the rear end. Mine came out pretty easy. I hear they don't always do so. The new axle seal kit comes with a new C shaped ring (circlip) for reinstallation.
Upon reinstalling the axle, you just simply push straight in on the CV axle. It collapses as you do this, but that's fine. One good jolt, and it popped right back in.
Reassembly is just the reverse. But when you install the brake shoes, you are supposed to use a brake shoe resetting gauge. This measures the OD of the brake shoes, and you set them a certain width smaller than the rotors. I forget the exact width right now, but I have it somewhere. Doing this step would eliminate trial and error of getting the rear brake shoes adjusted right later on, when it would be hard to get to the adjuster. If you don't have this tool, then I would suggest adjusting the brake shoes wider, until the rotor just barely will slide in over them, and then back them off just a little bit.
In my case, the truck was rusty, so I cleaned, primed, and painted all of these parts while it was apart. That took me forever, but it came out pretty nice. If I had to do it over again, I would have just bought a new brake backing plate, since installing my shoes made a mess out of the paint where the shoes contact the backing plate, and I had to grease those contact points.
The Ford manual calls for throwing out the old axle retaining nut, tie rod and ball joint nuts, etc. This is because they are metal nuts with nylon threads in the last thread or two (nylon lock nuts). I didn't see the need to do this after looking at the parts, and I understand that Ford's goal is to keep the nylon new, so there's no chance of these things backing off and you losing control of the car. I decided that I would just add some loctite to the factory nut and reinstall. There's still some bite to the remaining nylon, and if it's short, then the loctite should bridge that gap. Plus there's torque on them too, of course. I'm not sure why Ford calls for replacing the axle retaining nut. It looks solid and beefy to me. I doubt any mechanics replace it either.
I had to buy a torque wrench big enough to handle the 211 ft lbs that are required to retighten the axle retaining nut. My existing 1/2" torque wrench didn't go that high. A new SK torque wrench (nice) set me back about 150 from tooltopia, with free shipping. That website is a great place to look for tools. I bought the slide hammer kit, extension, CV fork, and axle rethreading kit from NTX tools, and they weren't so fast at all. But they were the cheapest on those parts. Because my truck was rusty, I thought I would have to buy the axle rethreading kit (chase set) for the 24x2.0 mm axle threads, since mine were heavily rust damaged. That didn't really do me much good though, since once cleaned off and chased, there really wasn't much left of the threads anyway. Just removing the axle nut would have probably done me just as much good. So just to review, here's what I had to buy:
New rotors, brake pads, parking brake shoes, hardware kit for shoes, hub bearing kit, axle seal kit, gear lube and posi additive, and caliper lube.
Tools I didn't already have: 1/2 torque wrench (big settings), slide hammer kit, axle nut socket (35mm) set, brake resetting gauge, CV Fork and extension (unnecessary and didn't work out), and a few others I didn't need.
I bought the hub/bearing assembly from Szott Ford on Ebay for about 100 bucks, which was cheap. You can buy just the bearing and the snap ring (or reuse old snap ring, but Ford calls for replacement of it). All it takes is to have a large bearing splitter to remove the half of the bearing that ends up stuck to the hub itself. You use the bearing splitter to hold the bearing in the press, while pressing on the hub. But I don't think you can buy the bearing separately any cheaper than you can just buy the hub assembly from Szott. Online auto parts stores showed a bearing repair kit, which I assume is a bearing and snap ring, but they cost more than the hub assembly. Oh, and you'll need a 20 ton press for the whole job too. I would highly recommend taking the knuckle and new hub assembly to a machine shop or 4wd shop, and paying them to do this portion of the job!!!