Wonder if the balance is off. Unless it is ready to fall out the back it should not cause a vibration. Think of it as a cocked yoke? Also if you let off the gas at speed the driveshaft is still moving also, Frank any ideas?.
Hey did you change the flex plate by chance?
When I had this driveshaft done it was balanced, trans is fresh, new converter...it did this before I had driveshaft made, and before the new torque converter...