John I'm not sure on the bell housing,engine and trans are both 1971 dates. What are you thinking

That one happened to me. had a bad u joint and it deformed the rear pinion yoke at high speeds it was noticeable at low, it was not. I've got an automatic but the problem is possible (however improbable) in both types of transmissions.

That video is interesting. I'm suspecting it's my driveshaft angle or rear end somehow. I think that will be my next step. My third member is original to the car.

I'm having a similar issue but it's at decel around 75 to 80. I remember having the issue last time I had the Mopar 4 speed transmission in the car.
So mine could be trans angle or driveshaft so I wanted to switch to 1350 joints and I'm getting a shaft made by Strange Engineering to replace the 25 year old shaft I've been whooping on.