So I unplugged the Ultra Gauge and it hasn't died on me again. And the CEL came back on shortly after (for the same EVAP code I've had since I bought the Jeep), so apparently it was interfering with the computer somehow. Kinda disappointing that its causing problems, since it's a very handy little gizmo. Guess I'll call them tomorrow and see if I can figure out why it's doing this. Going to give it a week or so before I'm confident that was really the problem, but its looking good so far.
2003 WJ Overland 4.7 HO, Clayton Long Arms, Teraflex 4.5" coils, JKS adj trackbar, JKS Quicker Discos, 17x9 Eagle Alloy Series 185, 285/70/17 BFG KM2, cut fenders/bumpers