Iirc all of the 6 speed trucks had the 11.5 axle. Most of the 5 speed trucks ('03- early '04) had the 10.5 in conjunction with the "SO" engine. Starting in 2004.5 all diesel trucks came with the 11.5. If your truck has a sticker on the side of the valve cover or the valve cover cover (03-05) the has a torque rating of 555, 600, or 610 you will have the 11.5.