2003 Ford Explorer Thumping sound on deceleration / coastin

I have been hearing a thumping sound (almost like the sound you hear when driving on concrete - at the seams, but only one thump) when coasting after going 40 mph or faster. This noise does not always happen, and sometimes, even after I've heard it, if I turn, brake or otherwise just accelerate, it might not happen the next time I coast.

It feels like the whole car is jumping a little - it is more than just a noise. I jacked the car up and manually spun the passenger side front tire. When changing direction of the spin, sounds like a thump is coming from the transfer case - could this be related? I've noticed other inquiries regarding the same topic, but have not seen a possible solution yet (there was a suggestion to check universal joint, but the inconsistency seems strange).

Thanks for any insight!