rhythmic vibration

I had this problem also when I changed my rear end to a 8 1/4" from a 7 1/4" and my tranny to a 727 from a 904. I also raised the back end of the car a little and this put my drive shaft at the wrong angle You might need to shim the rear end if you have done any of these things. I just wish someone would have told me about this before I spent $1000.00 on crap trying to fix it and all I needed was some $15.00 shims.. your problem sounds just like mine did though check it out...