trying to find source of vibration

I had a limofied '64 Chrysler once which had kinda similar vibrations I think as yours.
It would shake the car at low speeds when accelerating.

This car also had a 2-piece driveshaft, and, as with yours, while the intermediate support rubber was shot, it turned out later the transmission mount was soft and mushy aswell.
Replacing this mount aswell cured the low speed vibrations. Might be worth checking into.