Someone may have put the wrong springs in it before you bought it. They might be collapsing a bunch with weight on them. Give these guys a call and tell them what you have and what you are looking for. They are a good company to deal with, and most of their stuff is made here in the USA.

http://eatonsprings.com/

I have a set in my Chevelle, think I'll remove them and see if it get's rid of the wheel hop. Raising the rear changes the angle of the upper control arms. When the control arms are higher at the frame mount than the axle mount you have a good chance of wheel hop.