Differential or wheel bearing???

Put the rear of the car up on jackstands (do this safely because you'll be under it), start the engine and put it in gear, hopefully both wheels will spin, if not you might have to back off the brake adjustment if one wheel doesn't want to spin freely. Then lay under the car and take a long screwdriver or wooden dowel with one end pressed against your ear and the other end against the suspect bearing, you'll be able to tell if it's a bad wheel bearing or diff bearing by listening to it, one will be louder than the rest.