sounds like a set screw missing or loose from one of the center drive cups... easy to fix and cheap also... mine work themselves loose occasionally.... easy way to check diffs front and rear is to turn one tire at a time.. If the opposite turns in the opposite direction, your ok... then roll the truck forward and back on the front wheels only and see if the rear turns or the driveshaft turns.. that will tell you which drive cup may be to blame.. repeat with the rears down and the front up... hope this helps..