2000 Blazer 4WD, 120,000 Kms. Began hearing a rhythmic thumping last night while slowing for highway exit. It was tough to hear where it was coming from. I was pretty sure it wasn't one of the hubs as I've been there and it wasn't the same sound. I got it home, put the rear on jack stands fired it up and put it in gear. Thump, thump, thump as the wheels spun. A hand on the rear diff feels it but it could easily be transmitted from somewhere on the driveline. It does not thump in reverse. The thumping seems to reduce/stop when I stopped either wheel manually. The U-joints feel solid.

I'm narrowing this down but am a bit stuck. Any thoughts?

Many thanks

 

Welcome to the forum.
I'm not sure what the problem is, but I'd remove the rear diff cover and inspect the gear set. Put the trans in "N" and turn the driveshaft to see all the gear teeth on the pinion side gears.
Catch the diff oil and run a magnet through it.

 

It was the rear-end. Totally shot. Parts came to about a G.