When the differential starts going bad you're going to hear a growl coming from the front of the vehicle that sounds almost identical to bad wheel bearings and the faster you start going the louder the growl gets. The sound will change once the differential is about to give out and you will know at that point that it's a ticking time bomb, once it blows up your vehicle will be immobilized, you will not be able to push it. Good thing it happened to me 2 houses up from my own home and I was on a hill. I had to put a car jack underneath and lift the front in order to be able to move it at all.

Bought a transmission from Ebay for 310 with 43k miles on it, cost me 180 in freight charges from New Jersey to Illinois and got a mechanic to put it in for 250. The engine on these cars are the best ever built but those trannys are notorious for going out after 100k miles, a lot will depend on maintenance from previous owners and how they drove it, my personal opinion is that the engine has too much torque for the 4T65E Trans.