Do the things that are free first. Pull the rear brake rotors and check the parking brake assemblies. Check the half shafts that go to the diff, lift the rear off the ground and lower onto jack stands. Make sure the parking brake is off and put the car in gear, then grab the wheel and turn it back and forth. Try to see if there is play anywhere.
This could be a ton of things. Hell it could be that the rear driveshaft cover plate is loose.
Performance one step at a time.