Another drivetrain vibration.

Almost sound like there is a little too much pinion angle because the vibration decreases with increased load. Perhaps coasting smooth is because of no load at all.

Double check the angles now after everything "settled in". Should be with driver weight, but I can't imaging it would make that much difference. I guess it's a drive on lift? (Had to ask.)

Check U-joint condition and make sure they are installed correctly and clips properly seated.

Too much heat and long welding runs on a brace can tweak the housing, but can't see how it would cause a vibration.

Make sure the joints don't have any lateral play, especially at the rear end yoke. They have been known to spread from abuse.

Check tailshaft bushing.

Solid or poly engine mounts? Poly trans mount will transfer a little vibration, but can't imagine it being worrysome.

(I had a bad vibration once from an improperly lightened ring gear which caused an imbalance there. Drove me crazy.)

Enough fore-aft slip spline yoke clearance in the tailshaft static & throughout suspension travel?