If you go to the roundy round section of an AFCO catalog there is a lot of info on rear shackles and the effect of different setups. Well worth reading. If you have no rear lift, I suspect you have too stiff of a set of SS springs, poor shackle setup, bind, or some or all of the above. Too stiff on the shocks could be a culprit, although with SS springs that is very hard to achieve due to their very high leverage. The only other thing I could think of is spin at the hit(I didn't watch the vid, so,,,,)
I put a set of 002 and 003 springs on a 3300 lb Dart I am building and used stock shackles. It is stiff as all get out back there now. Not what I wanted. I should have bought the ones for a 2800 lb car instead. It is much better to have a low spring rate and control it with shocks and clamps than too stiff a spring, which can't be fixed without replacing them.