Clunk in rear end on bumps and the first or second up shift.

Hey folks, the dealer can’t get me in for a couple weeks and my truck is making some concerning noises, wondering if anybody can help me diagnose.
22 6cyl Auto with 17.5k miles, started making a clunk right before the first upshift when accelerating and makes the same (I think) clunk going over some bumps at any speed. I can’t replicate it by jumping on the back bumper or door sills, when driving I can only replicate it by accelerating semi hard intentionally to force the pre-shift clunk, it doesn’t happen consistently in regular traffic, but it’s getting worse even with gentle driving. I already torqued my U bolts to spec, my skid plates are all tight, nothing in the driveline that I can see looks out of place, and as far as I can tell my leaf springs are all in good shape. Anybody got an idea where to start checking other things? Thank you.