what does turn? Can you see the center drive shafts (from trans to the diffs) turning?
The drive cups (input cups) to the diff have 1 set screw holding them to the diff's pinion gear. If that screw is loose or missing, the center driveshafts will turn but not spin the gears in the diff.
If the center driveshafts are not even spinning, then the issue is likely in the trans.
Make sure none of the drive pins (which go behind the wheel hexes) are missing.