hello,
I own a 2003 tahoe 1500 4x4 with 4.8l, an auto tranny 120K miles. I recently had the front diff rebuilt new gears and bearings, but after towing a cargo trailer from Wa to Wi a gear whine developed while underload in 1 thru overdrive and during deceleration. it'* louder at higher rpms when the tranny first downshifts then tapers off unill it downshifts again. in park there is no whine, in gear with the brake on there is no whine. The tranny started to slip a bit while towing over the mountain passes, dosen't slip if not towing. It shifts normal in standard but if in tow mode it shifts hard and clunks into gear. I know the tranny needs to be checked out and possable new clutch pack/bands, but what would cause the whine? The rear diff or tranny? any thoughts would be great.
thanks

You said the front diff was rebuilt, but in the end you asked if we are thinking its the rear diff? Which one was rebuilt now?

First off, check your trans fluid while hot. Is the level ok? Smell the end of the dipstick, does it smell burnt? Wipe the dipstick on a WHITE paper towel. How'* the color look? Pinkish/red? Or black?

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.

sounds like you have a pretty good idea where its coming from, especially if the fluid was never changed and it was in a flood. sometimes when the bearings go it takes out the seals so you could inspect the diff front seal and around the bottom of the axles for oil. my guess is new bearings and seals for the rear

You might also look at your front bearing/ hub and cv half shafts. I have had whines coming under different load conditions, and ended up being my cv half shaft.