I don't know... not everybody runs the highways that much. The shape of your own neighborhood probably has more bearing (pardon the pun) on it than that. My truck was from up north, so it had a little rust on the bottom. I noticed that when I did the right rear wheel bearing, there was a lot less rust on the parts, corrosion damage, etc. Even some of the factory paint was left on the knuckle. Somehow, the right side got less corrosion damage. I would imagine the left side failing first may be related to that.
When I took apart the rear wheel bearings, I noticed that BOTH outer races had pitting damage, but only in about 1 part of the race that was perhaps 1.25" long. Strangely, only one bearing seemed really bad. The other no doubt would have ended up bad, but wasn't making much noise yet. In either case, I have to wonder how exactly an inner bearing race could end up pitted. Those races are extremely hard. And why only in one spot, and not all the way around? One could almost think that the truck sat in water up to the bottom of each wheel bearing. Or perhaps somehow snow and salt got in, but only up to that level.
The CV axle has a dust seal pressed onto it, that seals the cv axle into the knuckle housing. This seal would prevent the bulk of contaminants from getting at the back (and vulnerable) side of the wheel bearing. My seals were pretty cracked up. $7 each from Ford, I bought 2 new ones when I did my job. I would suggest that anybody doing a rear bearing job strongly consider doing the rear axle seals and the cv axle dust seals at the same time as the bearing job, since it's so labor-intenstive to get that far into it.
By the way, the rear axle seals have been redesigned several times by Ford, and the newest seal is a one piece design. The original seal was 2 piece, one on the cv axle and one in the rear end housing. The new design looks much better to me. One of mine was leaking anyway, so I went ahead and did both. $25 each from Ford. I lucked up and found the new seals (along with a pinion seal I didn't need) on Ebay for $20 for all 3.
While I had the axle seals out, I stripped, cleaned, and repainted the cv axles, knuckle, etc. I pressed in the new bearings, and also spun the axle shaft bearings (in the housing) with my fingers. They seemed good to me. Nice and smooth. Pressing in the new seals is supposed to take a special (long) pressing in tool from Rotunda, but I used a 24" block of 1x1 wood, and bolted a seal driver to the end of it. Then I whacked it in good, many many times, with a mini sledge hammer. That worked out fine. By the way, you can knock out the cv axles from the housing using a 10" 1x1 block of wood and a mini sledge. They pop right out.
Jono667, it's doubtful that a jiffy lube place would know about or honor the TSB on changing fluid viscosity. But if your rear end isn't making noise, then it probably isn't a huge deal. Synthetic is REQUIRED though, so make absolutely sure they put that in at least. If not, they should eat that. The synthetic fluid is about $15 per bottle to buy. And it's stamped right on the rear end housing that it takes synthetic only. If you have a posi unit, they should have added friction modifier as well, or you'll get popping noises when turning. My stupid truck didn't have a posi. 3wd I guess. There's a sticker in your door jamb. The non-posi axle codes are something like 45 and 46, and the posi code starts with a D, like D04 or something.
