other thing is if you spin one of the rear tires does the other one spin in the opposite direction?
If yes(and all the driveshafts are connected) then the issue is probably either the bevel gear in the diff, or the crown gear.(or depending on how the gears are connected a clip, or grub screw might have came off)

If no and they both spin the same direction then the diff is locked

In no and the other doesn't spin at all then you have to take the diff apart as the spider gears might be trashed. (or depending on how the output cups are connected a clip, or grub screw might have came off)