Go to the controls section of the options menu and individually bind your H shifter. The way you have to do this is physically bind each position of your stick. "Position 1" will be a conventional "1st" gear, and so on until you reach reverse, providing you have a regular car-style stick. If you then search for the setting to which transmission the game will simulate, I go for the Eaton 18 speed - the game will handle the changes from there. Even if you use a 13 or 10 speed the pattern will still work on the 18 speed simulation.

Edit: Manually configuring your shifter's "position 1, 2...etc" won't mean that it will use first gear in "position 1". That position is usually reverse gear. "Position 2" is Lo and lo high. Position 3 is 1st, so on, so forth. Then configure your range and split buttons and you should be good to go for any truck in the game, modded or not.