your rear suspension is not working. Tire spin? too tight? It is not even bumping against full stock extension. Rear shackle angle is not the problem (yet). Your springs are not moving. Traction? Sort the rear problem out, then look to worn out junkyard front shocks, right now your problem is in the rear.
When you get it sorted out, I have a new set of 65 and 66 SS springs, new. For a 3000# auto, 125# spring rate.
