Hey,  Yes its a dull thumping sound, almost like something is too tight and binding . 

I think a fluid change is the first port of call - inexpensive and relatively easy, with a small pump (no drain plug).

Depending on how long fluid has been there, change it regardless. I would have to hear it, or feel it. But, I think it's a worn or loose piece that transfers vibration to the body/frame through a rubber mount or suspension part. 

My '75 was making a snapping noise in the rear end when turning.  I changed the diff fluid and added positrac fluid.  I forgot about this until this post and I realized it was not longer making that noise.

The first time I encountered this was after a 150 mile drive to Donnington Motor Circuit, in the UK, on a motorway (freeway).  As we turned into the circuit it sounded as though someone had taken a hammer to the rear end somewhere!  It was finer while we were going around corners, but after a while on a straight road it would bang again when we turned, although not as loudly as the first time!    I did the  the fluid change and the problem was cured.    

Given how expensive, and difficult to repair, some rear end issues are, a fluid change is a quick and cheap fix!👍

Wonderful! The easy route is the first route. Glad it worked out and problem had been rectified.