Many TJ's were pre-wired for cruise control, even if they didn't have it installed, with the plug ending under the hood near the brake master cylinder as a pigtail that's not connected to anything. Makes it really simple to add factory cruise if you wanted to.
Since you have a '98, that's the same as mine. I'll guess you have 4 wires going into that plug: Black, Blue with Red Stripe, Green with Red Stripe and Orange with Red Stripe. I had to wire my cruise plug on mine.