If you are going to just run 38's i would stay with the 8" rize and not add the 2" spacer as the rize lifts normally sit about an inch higher than other manufactures lifts and you will clear the 38's fine. But if you want to go with 40's then you will need the extra 2" and going with the rear setup like Pahrump has is the way to go. Also if you go up to 40's you are probably going to want to regear since you are going to be in the sand and that will really tax the truck and tranny with out it
