Once upon a time in my youth a street and strip Olds powered B modified production 55 Chevy with a B&M Hdrostick.

Had tendency yo blow rear U joints and a what felt like a drive line generated vibration at 70 to 80 mph

One day jacked rear of car and placed our standard (yes I know) cement foundation bricks under the chassis, crawled under car and had my friend start the car and put it in gear.. Living on the edge,,,we are all lucky to be alive.

The rear U joint looked as a Mixmaster the way it was going up and down. Turned out that I had likely at one time tweaked the pinion flange in one of my U joint failures and it was running the rear of the drive shaft in an ellipse and not a circle. Replaced flange and was well.