While apart, I pushed and pulled on each joint and flange. They all felt perfectly tight with no runout. The transfer case and differential did, however, have a lot of lash, but no worse than any other time I've tested them.

When I replaced the center bearing, I had the forward rear driveshaft completely out and both U-joints felt just like new. I did not disassemble the U-joint at the diff. I only shook it while attached.

I was 100% confident that replacing the badly worn center bearing would be the final cure, but somehow, as bad as it was, it wasn't the cause of the vibration.

I've had U-joints go bad before on other vehicles, but they have always felt wrong when checked by hand. They either had movement within the joint, or they were overly floppy with the flange unbolted.