1966 c20 rear end wobbles and shakes whole truck when turning in reverse? Help

[inkfamous13]

Got this 66 c20 front end was sketchy at best, new tires and wheel balance has made that better, but when im backing up especially when turning wheel and in reverse whole rear of truck wobbles, shakes whole truck even so have to push gas pedal like going over a large bump? Its wierd. Could it be pan hard bar is not right? Shocks? Rear end. Trucks got some large lowering blocks under the rear trailing arms could this be throwing of alignment of bar? Any help is greatly appreciated. Thanks