Ok I have a question now ( never been great with rears) I have axle code 45 if I dropped a 3.73 or 4.10 it'll still be open right? What do I need to go limited ?
You won't find an OEM 4.10 for the 2002-2005 trucks, to do that you'd need to regear the one you have already. You can find 3.73 limited slip OEMs from a junkyard from time to time but you won't know if it's a "howler/growler" until you get it in unfortunately. A good place to search would be car-part.com, do a year/make/model search for explorer/mountaineer/aviator. But if it were me, I would either regear and replace all the bearings/seals on the original one or do that to the replacement to ensure its in good working order.
As far as limited slip units you can put in the standard ford trac loc carrier, but you would get a better service life out of an Eaton TrueTrac (gear driven limited slip, no clutch packs to wear out). Given the expense of regearing you might as well do it right and go that route.
2 things you need to keep in mind:
If you get a junkyard diff, you need to remember that to accommodate the traction control system they started taking the ABS tone rings out of the differential in 2004, and as far as I know all 2005's don't have a tone ring in the diff anymore. You'll need a non-traction control diff from a 2004 or earlier for it in order to to maintain ABS compatibility. Obviously this isn't something you need to worry about if you re-use your diff that you already have in the truck.
If you put in a different gear ratio diff, you'll need to get an SCT Xcal tuner from Henson Performance or elsewhere so the truck's computer gives the correct speedometer reading and correct shift points.
As far as prices for all of this, you can get the aftermarket 3.73 or 4.11 gears from Rockauto for around $130 dollars for the set. You'll also need a seal and bearing kit, get a "master overhaul" kit, which I think also runs around $130. Rockauto also sells the Eaton Truetrac Limited Slip unit for about 550 or so if I remember correctly. That'll put you at $800 in parts if you use the 5% forum discount code.
After that, you can remove the diff yourself using the instructions in this thread:
http://www.explorerforum.com/forums/showthread.php?t=318076
Take the diff and your parts to a good driveline shop in your area that does ring and pinion gears, will probably run about $300 or so for them to install the gears, bearings, and seals. Then reinstall the diff in the truck, fill with gear oil, adjust the computer with your tuner, and perform the 500 mile new gear break-in procedure. After 500 miles, drain the fluid, refill with fresh fluid, and you're good for a long, long time.
