Strong vibration getting worse - need thoughts

[Vitamin C]

OK, this totally blows!
This is my 70 RR
History: All about 10 years ago (been sitting for over 8 year) I rebuilt the motor (balanced as well as flex plate), new TCI Street Fighter 727 trans, new poly mount, same drive shaft, u-joints are solid, stock 3.23 8.75 rear.
All work done listed above has less that 1000 miles.
Always had a mild vibration on mid to freeway speeds.
I just rebuilt my rear and installed a rebuilt 489 case with 3.91 gears, this really made a difference.
Now it seems the vibration is getting stronger as the more I drive it.

My thoughts:

Sounds like the tailshaft bushing is going. Yolk is sloppy, moves a bit side to side. Something not right.

Possible causes are unbalanced driveshaft (I am dropping it off at south bay driveline this week to check and balance if needed.

Another concern is the yolk splines are not enough in the tailhouse, seems to only be in about half way. I read that only 3/4-1" should be left once inserted, does that include the rubber portion of the seal or edge of case?

Hard to tell the pinion angle but I am thinking this is not going to be too bad. I do have the Hotchkis 1" lowering leaf springs. It's really tight under there especially with the exhaust and H pipe.