Nothing really. There's another sensor in the driveline somewhere forward of the rear t-case output. It's purpose, as near as I can tell, is to make sure either the transmission output and the t-case rear output are spinning at the same speed or that the rear and forwards t-case outputs are spinning at the same speed. Apparently changing the speed signal coming from the rear output made it in conflict with the front signal and that caused the check engine light.
Never did nail down the reason or location of the other setup. I know on '99 GV's this wasn't an issue but on '03's it was. On our '02 XL7 I decided screw it and we'll just throw it in the dash and cross our fingers.