THere is a tool which might help track this down... There is such a thing as a mechanical ' tach'.... this is a spring wire deal which is held on something and the tube around it is adjusted to get the maximum swing of the wire.... then you read the occillation rate from the side... this tool could be held against the dash for instance....
You can figure the wheel turning rate from stuff... and if the vibration rate is approx 3 times the rotation rate of the tires.. then you can figure the problem is driveshaft related.
Some changes were made ( like so many other things ) in the flex discs.... some cars in the corrected mode have different ' stiffness' discs on the front than rear.... when they originally had the same ( if memory serves me ).