Driveline vibration!!! I need help!

I have the same thing with my Duster. It actually did it before I changed the engine and rearend. Still does it with new engine, driveshaft, and Dana 60. the transmission is the only common denominator. It is not a tire, it is in the drive train somewhere. it is not constant, it is speed related, 50 plus and goes in and out in a rythem, 1 second smooth 1 second vibrate, smooth, vibrate, smooth vibrate until I slow down under 50mph. I am stumped. Is this what yours is doing?