I'd try it without the shocks for a real quick drive to make sure it's the springs that are giving you the shock. Sounds like you need a much lighter spring out back!
In the old days they used charts like this and you would then pick from a supply of 50 different springs:
http://1969pontiac.com/1969%20RPO%20...20Weights.html
...but now we get 2 springs, big block and small block! Which means the people that needed any of the 48 other springs wind up running something "wrong" for their truck.