Engine Vibration at highway rpm/speed

You kindof have to run it with the D-shaft off and the yoke in;else the tailshaft is unsupported and it will whip around, screwing with your head. I have done this several times, with the rear of the trans centerline, a tiny bit higher than the front. This has always worked for me.

Another thing that has worked, is to drive it up into the vibration zone, and then change transmission gears to increase the engine rpm. If the vibration remains unchanged then you have eliminated everything from the trans input and forward.

Tires also have to be round,not just in balance. Else they hop the axle up and down relative to the road. The shocks pick it up and transfer it to the chassis. This can excite the A-body's natural tendency to oscillate at about 55 mph.
I run reduced tire pressures, in part, to help reduce this.
This body vibration is not actually felt but is heard everywhere inside the car with some ares louder than others. It's kindof a ; whooo-whooo-whooo sound, starting quiet,getting louder, then decaying; and repeating.If you put a stopwatch on it, you'll find it oscillating at about 60 or so cycles per minute at 55mph, and it doesn't seem to change progressively much with speed. It just changes volume/intensity, mostly.