Either can be made to work with mods - the issue is the gear ratio may be different than what you have in your truck, requiring a front diff gear swap as well. The 3.0 should have an 8" diff, the 3.5 is either a 9" or 9.5" depending on year. It will require the driveshaft to mod the end to match the rear if you use the one for the 3.5, the 3.0 should be the same as what you have. If you swap the front gear set out, get a locking diff from a starquest - it is slightly smaller than the rear diff carrier of the truck rear and a perfect match for the front. Since both rears are leaf spring, it should bolt right onto the leaf springs, but it is wider than the truck diff. It is easier to swap the whole axle than swapping the disc brake parts over - it means rebuilding the whole axle, and using the axles and brake parts and hoping they fit the truck axle housing.