When drive shafts are manufactured, they are not perfect, and like wheels, they are balanced with small weights. Unless removed from the car or disassembled in any way, (or bent by accident), they should not go out of balance.If the shaft is dismounted, make sure all pieces are marked in order to replace it in original alignment.
When I restored my '59 coupe, I had to have the shaft (including the the U-joints) balanced as a unit.
I would think your problem is as stated in the previous replies. Ed