You don't want lower gears, you want higher gears. Only choice with the stock rear is to find the 3rd member from the '68-'72 3/4 ton Chevrolet, V8 automatic, with the 4.10 ratio option. It's a direct bolt in to your existing housing. For anything higher than that, you're going to be looking at swapping the entire rear axle housing. In that case, the Dana 60 found under '80s-'90s Ford and Dodge 3/4 ton vans is right at the proper width. They usually come with a 3.55 ration. Word of warning, those are almost always a semi-float rear, and are not as strong s the full-floating rear that your truck has.
