Welcome to the site! Usually, a 31 on a 4x4 and 30" on a 2x4. I have 2wd and a 2" lift to give my 31's plenty of room. Each X is different, but usually a 31" can go on without much fuss. You can always raise the front a bit by using the torsion bars. Many members have a 1-2" lift by using the torsion bars in front and some new shackles in the rear. Use the search engine for "torsion twist" and "war153." That's the model number of some great shackles made by Warrior.