Thanks. I kinda agree with you.
Yeah, it's a thump-thump-thump from the driveline, at approx 3x wheel speed. After going through engine mounts, tranny, driveshaft, etc; I feel pretty strongly it's not those. Maybe the u-joint, but it's fairly tight, and the fail mode for it seems to be a constant vibe at higher speed.

I did reverse the axle shafts a few years ago, to see if I could notice a difference, but no change. Axles feel tight when flopping them around by hand, but that probably doesn't mean much.

One thing I did notice; it goes away when the car is cold, the longer it's driven the more noticeable it becomes.

Doesn't seem to have gotten much worse over the last 75k miles, so it's gotta be a slow wear issue, like a u-joint or CV joint.
I thought it would get worse, then I could find it, or justify buying new axles just to try.

Maybe I'll bite the bullet and pop new ones in, getting kinda tired of failure in locating the problem.

DG