As a 12 year Ford dealership technician and a senior master tech for 2 years, I would like to chime in. ALL 3rd gen Explorers had rear diff problems from day 1. Both 3.55 and 3.73 ratios, open and Trac-Loc equipped, 2wd, 4wd, and awd. The front diff is never a lsd from the factory on an Explorer or most Ford vehicles for that matter. Back in the day, when these first came out, we used to replace the diffs with complete drop out diff housings until Ford came out with a new gearset. I saw 2 or 3 very nice and very low mileage Explorers get bought back by Ford when they came out due to these issues. The first problem, was a whine noise while driving at speed, then the chatter on turns noise, now mostly I see bearing failures and whine noises. Even the ones that only have a whine noise usually have bad carrier bearings. Ford only sells a new ring and pinion with all of the bearings as a kit. The parts are fairly cheap, most of the money is in labor. Usually in the $1000 range just for that depending on shop labor rates. Parts are usually around $400-$600. I have personally done a complete rear diff repair in a hour and a half. That is driving the truck onto my lift, removing diff, clean everything, completely replaced all bearings and ring and pinion, reinstall, and test drive.
I am also not saying that it is for sure your diff, as we all know rear wheel bearings can make some rear noises as well as e-brake shoes braking loose from the backing plates.
On my personal Explorer, it started life with 3.55 open rear diff. I got a used Trac-Lok carrier, put new bearings on it, pulled in my 190K mile truck, removed the diff, replaced pinion bearings, reused my ring and pinion, and reinstalled it. Now I have a lsd with all new bearings and seals, with the original ring and pinion. Most shops might not do this for you since it might make some light gear noise, but on the other hand, mine was not broken. I just did it as maint, and an upgrade.
Very long story short, If you need a new rear diff, a Ford dealer is going to sell you a complete overhaul, gears, bearings, seals. That is the most cost effective way Ford can get the parts. I have no problem with used parts, I am a cheapa**, but as long as you keep the ring and pinion matched set together, and no teeth are damaged, you should be fine with used gears, but unless you can do the work, or know someone who can, you will have a hard time finding someone to put them in for you. Some dealerships may just try and sell you bearings, but I doubt it, plus Ford recommends gears in a tsb still. And keep the same ratio on non 2wd vehicles.
