I agree with arctic your tire pressure is way too high in the rear. Drop it down to 50 lbs when unloaded the ride should get a lot better. I took the blocks off the rear axle on my F350 DRW and added air bags. Dropped the lowered tailgate hight by 5 inches truck now sits level. I adjust the airbags to load. Side benefit is the headlights are always correct no matter what load. Depending on tire width you may be able to grind the stops on the knuckles and get a bit more before the tire rubs the front spring.
1999 F350 AUTO 4x4 CC LB DRW 4.10