55.2-59 Truck bed bouncing

[popeyestruck]

Took the truck out for a shakedown and the rear end is bouncing pretty bad. It rolls good while pushing it . Seems to be worse once the clutch is all the way out and not as bad with the clutch partially pushed in. Would an out of balance driveshaft be that noticeable? I didnt mark it when I took it off because I figured with new yoke and u joints it would be off. I rebuilt the rear end ,rear brakes , new shocks , rebuilt transmission. What might cause this?