When you replace the rotors, wire brush all the rust off of the hub where the rotor mates to it. Also clean the mounting face of the rotor if you're putting the old one back on. You should also check the rotor for runout. Excessive runout can cause warping too. Runout should be no more than a few mils max.
2000 WJ 4.0L 42RE 242 TC
190,091 miles & counting