65 Valiant Vibration /6 and 904

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 !