vibrating driveshaft ??

[Viator Trudeau #8697]

My friend W/ a 1951 cad. 4 dr. sedan has a vibration at 55 MPH.
& has had the drive shaft out & balanced, but there vibs are still there. Any thoughts out in CLC land on where else the problem may lie. thanks

[Ed Mobley]

See my post regarding lateral alignment of the driveshaft.  Alignment of the drive shaft can also be affected by a worn rear ball joint.  As such, Im going to replace mine.