Driveline Vibration

The vibration I would get would come on at 37-40 mph and get worse the faster I went and it would shake the whole car when slowing down.
I had a new custom driveshaft made and ballanced and was still getting the vibration after taking it back to the driveline guy twice and after I checked the pinion angle 3 times, got new tires, had the wheels and tires ballanced, ect..
I was just about to pull the tranny and have it rebuilt when I noticed the old 904 I pulled out had the weight on the tailshaft and the 727 didnt. I tryed to swap them but the 904 holes wouldnt line up. i found one off a truck in a junkyard and the vibration went away.
I don think that it is standard equipment, when I was looking for one only about 10% of the cars/vans/trucks I looked under had them and most of them were 904's.
If anyone knows what cars they came on and why some have them and some dont I would be interested in finding out.