I added Bilstien shocks and PacBrake air bags. I did not need the bags my truck sits level when hitched. The shocks and air bags did smooth out the really rough roads. I run around 25 psi in the bags when loaded with 4,500# pin weight.
Proper air pressure in the rear tires can make a huge difference. If you have a Dually weigh your rear axle and divide by 4 that is the weight each tire is carrying. Use the weight/air pressure chart associated with your tire size and load rating to determine proper air pressure. Add 5psi to what the chart shows.
Too many think they should run 80 in the rear duals and they have a rough ride and wear out the center of their tires.