Have you got video of the car running? It sounds like the rear shocks are bottoming out and unloading the rear after the initial hook. I would try a more adjustable shock. I have seen the same problem on a few cars. Do you have the extra leaf on the passenger side?
Here is a link for some info.
http://www.moparmusclemagazine.com/t...s/viewall.html
Hope this helps.