I'd suggest that you beg, borrow or rent a code reader. Your local parts store may be able to help you with this.
There are WAY too many possibilities for us to make any reasonable guesses as to what you should do. You MUST have this code.
When you get the code(s), post the EXACT codes here for comment; not just a description. With OBD, the devil is definitely in the details.
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!