90 truck suspension

I purchased a 1997 truck in Jan this year,after sorting out the body and other bits and pieces I am aware that the suspension needs sorting. The back end jumps around all over the place and I have changed the shocks to monroe gas units, the old ones were completly shot,It has however made little difference, Could it be the springs?.It is used mostly on tarmac but sometimes in wood land which is okay because of the low speed, I am sure some one has the answer, Help please.

more likely to be the bushes somewhere. check all of them at the rear end.