Whoa whoa whoa....

If you are rolling along at 80 mph and the vibration is there...and you put the car in N and the vibration goes away...

would you not want to investigate the converter/flexplate/flywheel balance? If the driveline was causing this the vibrato would still be there even in N when the car is rolling. Personally I'd think about finding a different driveshaft shop. It is straight or it's not. If not it needs to be retubed or it will never be correct.