Suspension improvements start and end with either Don Thuren or Sage Carli. They can do anything you want.
The rear is the worse in stock configuration b/o the mentioned springs. With the helper spring in place you only get 1-2 inches of travel and with so little weight on the rear well, it's not good. The front doesn't have much travel either and the weight of the Cummins, the stock shocks are overwhelmed pretty easily so your probably on the bumps when you hit those pots hole.