A certain amount of force is required to deflect a shock. Add a second and more force is required. And so on... 99-04 SDs already have a high spring rate which is resistant to movement so these require very little compression dampening. However they do require greater rebound dampening in order to slow the energy release of the springs following compression. Application specific shocks such as Bilstein 5100s and Donahoe Racing Signature Series feature very soft compression valving and a high flow digressive piston design as well as greater rebound dampening developed specifically for Superdutys.