Thumping noise on rear end of T100

Newbie here,

I have a 96 T100 4x4, extended cab with 210 k and just love this truck! I want to keep going. I have a thumping noise in the rear end (more so on driver side--no pud intended). It makes a thumping at speed at what seems to be a full revolution of the tire. It is consistent and synchronous. I've had a garage look at it and they mentioned a definite noise in the rear differential, though they can't tell or isolate on where it might be coming from. They mentioned it might inside the rear end train. I even had the tires rotated and balance again so I think that's not the issue. I bought the truck used and it always had a high rear end train noise pitch at speed.

Anybody experience this before or have any ideas on what the thumping noise might coming from before I take it to a dealer? I like to get some info (get some feel what I might be up against) before I take it to dealer and drop a load.

thanks in advance for reading and your input!

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 :thumbup:) 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.

FVZ-FE thanks! Your detail info was what I was wanting and half expecting! Yeah I figured it was going to be some bad news and like everybody wanting to hear something simple.

Again thanx's for your input!