I'm pretty sure the front diff is fine. It was just rebuilt in march by a good specialty tranny and differential shop and the sound isn't comming from the front like when the front diff bearings went out. It also shifts into 4x4 with no problems with no change in sound. I hear the whine more in the middle to rear of the cab. so I was wondering if it was the rear diff. or the tranny. I had the tranny serviced, fluids changed, tranny filter replaced, and front and rear diff oil level checked before the move so I didn't get to see or smell the old fluid, but when I got to wisconsin (2000 or so miles )I personally drained the tranny fluid and filled it with new tranny fluid. No metal, normal level, not burnt, still red. symptoms stayed the same. I haven't checked the front or rear diff oil levels yet, but there is no leaks and the rear diff wasn't hot after driving today.
On a side note; I bought the tahoe for cheap with 90k but no known maintenance records, and after owning it for the last 2 years I found signs that it went thru a flood. Durring the front differential rebuild the shop said the front diff was hashed, and the fluid was some of the worst they had seen. I'm sure the rear oil has never been changed ( I know I'm lazy) so I can only imagine what it will look like. I'm going to change it this week, however with the symptoms I described I was wondering what'* everyones best guess is to where the noise would come from and / or how to diagnose where the sound is comming from by myself.