Driveline vibration!!! I need help!

Check to insure that the Universal caps are held secure and are centered in the yoke on each end. On my drive shaft, I had to use 2 C-clips on each Cap, otherwise the caps would slide out to the C-clip, allowing the Universal to slide to one edge or the other. It was intermittant, but followed the same problem you are experiencing. Also be sure to inspect the Universals for wear. My 66 cuda completly lost all of the needle bearing on one corner, causing it to vibrate as well.