From what I gather Fbody driveshafts were made out of phase on purpose to try to stop vibration. IHad always had a vib in the car for as long as I could remember. Back in the 80s the vib got so bad at around 120+ that the the u-joint broke and the driveshaft went flying out of the car(after causing all kind of damage). I had to replace the shaft with another from a wrecking yard. It had the same issue. I lived with it for years because it didn't show itself until 80+ mph. Last year I pulled the driveshaft out to get it balanced. The yoke was toast and so was the tail shaft bushing( if one is rough you can figure the other is as well). New yoke, u-joints, tail shaft bushing and a balance job and she runs smooth to at least 115 now( as fast as I've taken it yet).