I see on their site they list a 4speed auto 518 (should be the same as a 48re from what I can gather), but want to be sure it will work. I see a lot of older 4x4 dodges running this setup, but no newer ones yet. I am running 35's on my truck with 3.73's and want to underdrive 4th for mountain towing. The gear vendors unit is not an option as I want 4x4 with my set up, and I need underdrive, not overdrive. Thanks, Jason...