65 Valiant Vibration /6 and 904

[Jeff Lemon]

55 MPH and above " only had it to 75 MPH " I have a vibration in the seats and floor not in the dash or steering wheel. Prior to my owning this car the ball and trunion was removed and a later driveshaft was installed. The u-joints and transmission mount look find. Could something else be a problem when the newer driveshaft was installed ? Tires are new and balanced correctly.
Thanks for any help !

 

[65_valiant]

D-shaft balanced?

 

[Supershafts]

I'd have the shaft balanced, if the shaft was converted to a stub and slip make sure the slip is in line with the rear weld yoke phase.