Jack the truck up and pop off the differential cover. Do a little inspecting to see what's going on. A little silicone and some gear oil is all it will cost you. But open it up and see if you have broken teeth, with the rear end supported on jack stands turn the driveshaft and listen for noise if you don't see anything obvious.
Ok. Thats one thing this truck hasnt gotten yet is changing the gear oil in the rearend.
Ever?
Oh well, it will get changed with the new diff assy you are about to install.
I just remember that rant thread about brakes, seals, and the inspector that didn't catch your lack of maintenance. I thought you checked and changed your diff fluid back then.


are you talking about?!?!