I have it too. Definitely rear end backlash whine. Same speed and lift off gas and it is gone. Get really into the gas and it is gone. Just coast with small amount of gas to maintain speed and it is there.

My dealer wants to go for a ride to listen so that they can think of excuses of why they won't fix it I am sure. Not that I really want these monkeys taking apart my entire rear suspension system and trying to get it all back in there the same way. These are very complicated suspension systems. Do you really want these five thumb dealer messing with your ride?
I will live with it, until it falls out on the highway behind me. Then Dr Z will warrantee it. 7/70 baby. And I have a spare speedo head.