The rear diff is mounted in the rear sub frame with rubber bushings, so you never hear the whine. With my car, I made solid bushings to make it have no more play in it. The combination of the rear diff being mounted solid to the subframe, and the subframe being solid to the rest of the car using these lockdown bolts, made it so I could hear the rear diff loud and clear at cruising speed on the highway. Once I made the subframe lockdown bushings out of Delrin, now I can barely hear the rear diff at all.
The difference is that I don't think anyone has mounted the rear diff solid to the rear subframe like I have. Yea, the polyurethane bushings for the rear diff would probably work better, but I already did mine solid, and I love the response of the rear end now.