Firstly,you have got to get the alignment right!!
You need at minimum 1 to 1.5 degrees of toe in on the back wheels.The rear hubs have a habit of wearing the fulcrum shafts just a little.When you jack the car up try to wiggle the wheel from front to back (like feeling a wheel bearing),if its got play its the fulcrum shaft (cheap to replace).Check the bolts on top of the diff,they may have come loose.Also I put track rods (radius arms) running backwards to solve the same problem,they can be made long enough so as not to effect the toe when travelling up and down.
