Noticed a tip in throttle harmonic whine at 60-70 mph last Spring and took it in to the dealer (7000 miles). They said they heard nothing. Over the course of the year it has gotten louder so I took it back with 17,000 miles and they could hear it and put a microphone on the diff and sure enough...it was whining more than normal. They ordered a new complete rear diff assembly and installed last week. So far no noise but I am taking it easy for 500 miles to avoid over heating while the new gears break in. The dealer tech drove it 15 miles on the freeway and said no noise.
I have my fingers crossed........