I thought rock in tire too, but you say it sounds like a loose rock inside a hub cap...

Rock in tire is a click sound. "Snick Snick Snick" sound that varies with speed. Axle bearing more like a like thump. If you jack up your rear end, one side at a time and listen for sound you probably find it. If you have safe-t-rack rear end then jack up so both tires off the ground.

Block your front wheels in either case.

Also, check for loose mounting bolts/nuts on rear shocks.