73 Dart Swinger Vibes and Solution?

Just Incase you don’t know this:
To measure for drive shaft length-
make sure you jack up the car by the rear axle and put on jack stands the axle as well. If you lift the car and the axle hangs your adding to the below 0 axle angle and the length of the drive shaft. I personally like about 1” exposed of yoke out of the back of the transmission but with the large below seal you need less exposed like 1/4” because the actual seal is inside the bellow about 1”.

again- just Incase you don’t know.
Also, I had that rymic vibration at a certain rpm. I changed my center chuck to a different ratio (3:23 to 3:91) and it is gone. Mine was more of a drone at like 2200rpm from my mufflers. Load and rpm change made all the difference.