The 4x4's are not available with the 20" wheels. When you order the 20's on a 4x2 they use a different rear end ratio (3.92) to make up for the height. I think 4x4's are also a 6 lug where the 20's are 5. So I dont think your truck would be able to have the stock rims. Im sure an aftermarket company makes something in a 6 lug 20". Hell I saw a truck this week with 24" wheels.
I just bought a 2003 Ram 1500 Quad Cab 4x2 5.7 Hemi Magnum 20" Wheels.