I would use the TNT frame stiffeners for the XJ and just add some length. They also make a belly pan for the MJ. You only really need to protect the vitals which the belly pans do. Or you would just add some 3/16 steel or make another plate in a area you are worried about.
2008 3.0L CRD WK LTD - Wifes, GDE DPF Delete Tune, Turbo Resonator Delete, 3" Intercooler, Addcoo Rear Sway Bar, 3.07 gears
2007 3.0L CRD WK LTD - Stage 2 Custom Spooling Tune, Go Fast Parts
2003 F250 6.0L CCLB - Gearhead Tunes