Anyways, it could be the diff, a bearing going bad in the diffs, the rear hubs, or the motor itself.
Try disconnecting the rear drive axles and running the car to see if the sound is still there. If so it's in the diff or the motor.
Try another motor. If you still have the sound it's in the diff. If it goes away it may have been the motor or possibly your gear mesh.
If it's the hubs just check for a bad bearing.
If it's the motor pull it apart and look at the armature. See if it shows any signs of rubbing the magnets when in use. Clean the motor and put it back together. Spin it by hand with the brushes out do see if you can hear or feel anything rubbing.
If it is the diff pull it apart and check all the bearings. Rebuild the diff exactly the way the manual says. Spin it by hand while out of the car to see if you can hear or feal anything rubbing or binding.