I also have a '96 t-100 4X4. I had the same issue at ~90K - whining noise at about 40-60 mph, along with the rhythmic "whumping" noise. It turned out to be a cracked transfer case housing and F-ed wheel bearings. I had to have the whole rear-end replaced - transfer case, differential, transaxle and wheel bearings.
In my case I guessed that the cracked case caused: 1.) a leak in diff. fluid, thus the F-ed diff. and 2.) bearings wore out because of lack of structural integrity of rear end.
It was expensive, so I hope you fare better. I took my truck to a garage that a friend recommended (not a Toyota stealership). Everything has been tip-top since. (150K+ and counting
) Except for the whining noise (reduced, but not gone altogether) - they said the noise was a result of wear on certain parts of the diff. Not a problem, just a minor nuisance.
