I had the same areas rotting on my '98. Had to get a new frame from Jeep and replaced the torque boxes. On Ahoragi's pics it looks like there is under coating that's dried and cracking, which lets in and traps moisture against the metal, promoting more rust. Whatever you do, avoid rubberized undercoatings like the plague. I used chassis saver on my frame and underbody, and sprayed eastwood inner frame coating inside the frame rails. And remember to drill drain holes in your frame so this doesn't happen again!