What would cause the 1 spark plug to burn up in a 1998 gr Cherokee 5.9?

Could be a number of things, check your compression on all cylinders and be sure they are within approx 25 LBS of each other. 150 LBS is very good compression to have. Check your fuel injector on #1 to be sure it is squirting fuel properly. That is difficult for most ot check, but give it a try if you can. You might have to have a Leak Down Test done to see that your valves are good. It could also have a warped, cracked, head, or bad gasket. Tell more about "Burn UP" What does the plug look like? Color, Tip missing, erroded, corroded ETC...