Funny you guys are talking about this, I actually pulled my drive line off of my 68 Firebird 350 HO 4 speed Convert, 36000 original miles, (the factory u-joints installed) Monday to repair the rear u-joint. My drive shaft is out of phase? I've had this car for 27 years, and it was my friends Mom and Dads car since new, so I know the history of the car. I brought the drive shaft to my work to repair it and when the machinist saw it he said it was messed up, and that it should be in phase, and recommended that we cut it and line it up, also the yolk shaft, on the front of the drive shaft was worn down in the middle, he said it was due to the mis-phasing and could cause transmission problems, because the drive shaft was wobbling. And I do agree with him. Well I am going to a car show this weekend and needed to get this fixed ASAP, so I have a 69' Camaro with a 12 bolt 4 speed in her and we thought that might work for my Bird. So my son pulled the drive shaft off the 69 Camaro, and when I got home last night it had the same out of phasing as my Bird? So, does anyone really know for sure why this is?? And if so why only on certain cars? And can this cause the yolk shaft to get worn down? My friend had a used yolk and we are installing it on my original drive shaft right now, but should I cut mine and phase it, or leave it and worry about the tranny going bad??