Drivetrain Vibration

This has to be driving you nuts, i know it would cause sleepless nights for me.
There is 3 different speeds of rotating assemblies going on.
Engine
Driveshaft
Rear axles.
i think you have ruled out engine vibration,and driveshaft.
Tight spot between ring/pinion
Bent axle
Wheel with excessive runout.

when i set driveline angle, i jack vehicle under diff and set engine
At level. Then i check how far up or down pinion is. I do it this way to simplify it in my mind.

I have also run vehicles up to vibration speed on my hoist, but it terrifies me. And it adds other variables into it, no load for one.