Firstly are you sure that the noise is emanating form the diff?
I say this because often noises from that general area result from the prop shaft hitting the transmission tunnel. I agree that this would probably be more of a rattle than a whine, but it might be worthwhile checking while the car is up on the ramp to change the oil. It is very diffcult to do from the top as you really have to remove the seats, some upholstery and the tunnel to check. Not something you want to be doing whilst on holiday.
On my 2006 Roadster one of the rear suspension hangers broke on the alp passes. That let the prop shift hit the tunnel on had bends. You can easily see that as the damper will be loose.
