The general/off-road valve code shocks (255/70 that is available in the 5125 series) work pretty good on the full size leaf sprung trucks, but there is an advantage on having vehicle specific valved shocks. The vehicle specific code for a leaf sprung, diesel pickup is 298/36. This means the leaf spring can do it's job better because the compression code (36) is less than the 50, and the rebound code (298) is heavier than the 255 which means it better controls the trucks’ rebound action and reduces bounce.
We have a lot of people that are comfortable with the 255/70 valve code on full size trucks. We do agree that even if not dead on to Bilsteins recommendations, the 255/70 being a Bilstein is way better than the standard white body twin or mono-tube shocks. Bottom line, in the 5125 series, the 255/70 is the best choice.
Hope this helps.