I've heard oil can help..redline makes one. from my experience whine is usually from the pinion setting in the differential. It won't harm you but annoying. I assume you have serviced the diff oil.? You could replace the rubber differential mounting. I think there was a lot of sound deadening behind the back seat, you might check that too. Are you sure it's a differential noise? Not in the axle or brakes? Noise can transfer.