I have been doing a lot of research on this subject and no one can really explain, or knows why on first gen Camaros and Firebirds why some of the drive shafts were out of phase, but everyone agrees that this is not the correct way for drive shafts to operate with out some type of wear or damage. On my Firebird, as I mentioned, the yolk is damaged, it has excessive wear in the center of it, kind of like an hour glass/coke bottle wear? Has anyone else had this issue?? I did replace the u-joints at lunch today, and they were fairly dry, but not totally shot where this would cause vibration issues. I also replaced the yolk that was wore. I'm curious to see if my transmissions rear bushing and seal are toast, and possibly rear tail housing, and also if there is very much lubrication in the tranny? I'll be installing the drive line back in the Bird tonight, and will check for "tightness" on the transmission shaft, seal, bushing and see if they need to be replaced. Also the yolk on my 69 Camaro with the out of phase shaft has the same wear on it's yolk?? Very interesting... Do all out of phase Drive shafts cause this problem?? Has anyone else seen this?? For the money, and ease of mind, I think when the car show is over I'll replace my drive shaft with an in phase one. Any comments are really appreciated. I'll let you all know how this turns out.