If you look underneath the truck you will see the drive shafts that go to the transmission. At the end of each driveshaft is a drive cup. Theese drive cups spin the driveshaft which spin the diff. The drive cup are held in place by small tiny setscrews also called grubscrews.If theese are loose, they will spin but supply no power the the wheels. I suggest that you locktite them in place because they come loose very easily.