Shift kits are available for both the AX4N and AX4S I believe, however, unless your transmission is already acting up and needs a rebuild, the labor for the install is not worth the effort. If you are having it rebuilt, that is the time to have the kit installed.
If you are interested in increasing the life of the transmission, I'd suggest installing an inline secondary transmission oil cooler. These are inexpensive and fairly easy to install. See the mods section at the top of this forum for information on this.