I was wondering about the U-Joints, myself, not entirely sure that was the case though.
I have rebuilt the transmission, swapped out the rear axle for one that is known to be vibration free (was in my 78 sedan, originally came from a 73 wagon as well though), new U-Joints, new transmission and motor mounts, and new tires with freshly balanced rims.
The only thing I have not have professionally tested yet, is the drive shaft. If that's not it, I don't know....
I have a shop who will spin balance my drive shaft for $70.00. Isn't that a bit expensive though?