Sounds like a problem I had a while back. The rear most bolt on the driver side rear lower control arm/trailing arm was the culprit. Somehow the hole in the bracket was larger than the one on the arm itself causing it to move. So I got a bigger bolt and slightly reamed out the bushing on the arm and that's all it was the whole time. Literally a $3.99 fix.
Have someone stand on the rear bumper and slightly bounce up and down while looking at the trailing arm bolt locations. There was just a little movement there but it translated to quite a bit overall. I'm not sure I would have ever found it without the second person.