Actually, most 2-speed rear axles are geared pretty deep. I have a 2 speed on my 1954 F800. It has a 6 series high gear and an 8 series low gear. The low gear is for getting started with a near maximum weight load. On later model trucks, they still run 5 series and 7 series gears. These are primarily work truck rears, designed for dumps, towing construction equipment, etc. These rears just make up for a lack of power.