trying to find source of vibration

hi guys I know its not an a body but still a mopar what im working on it a 1961 Chrysler imperial four door with push button trans and a 413 in it. the story starts out with the car had a really bad vibration but found out the center bearing was shot it a two piece drive shaft so sent out shaft to get a new center bearing put on, drive line shop called me and they told me that the second part of the shaft the rubber was damage so the drive shaft was junk so we just had a hole new one made, got the shaft back still 2 piece and put in car drove vibration was gone at 45 to 50 but still had a vibration at 20 to 30 mph, called shop they said we can recheck the balance of the shaft but they said they had a very good balancer had it done still only vibrates at 20 to 30 and after the smooth as glass check all motor mount trans mounts, and it doesn't matter if you step on gas pedal or just touch it 20 to 30 it vibrates im lost any help would be great