i think the poly diff mounts increase vibration and noise in the car.backlash is preset so there is only enough room between the gears for oil.if the gears are worn then the diff will clunk,could be the center prop bearing,that does wear out too.get under and get the rear wheels in the air and turn the prop and see if the prop turns a bit before turning the shafts,and grab the prop to see if there is any play.