my opinion is that it is the diff. You could confirm my taking the rear diff cover off and triying to rock the ring gear back and forth. You're trying to see if there is any slop or back lash between the ring gear and the pinion. If there is alot of slop, you might have to get the pinion nut tightened. This can be done without taking the diff apart but if you tighten too much, then you're in trouble. It's best to let a pro do it.
Also see if there is side to side movement of the ring gear. Side to side movement = loose or worn carrier bearings. BTW, the rear of the car should be in the air and the drive shaft should be locked. Also, you'll lose all your diff oil as well, so you may want to just keep driving it as others have said.
