Check this thread out:
Manual belt tensioner question... missing tensioner bolt? - JeepForum.com
The part numbers given there are good, as I've just ordered and installed them myself. Down near the bottom is a diagram you can print out and take to your dealer, and he can get the part numbers off of that to order what you need. I think everything cost me about $27, which included everything except for the pulley. The pulley they had listed was like $40 by itself, so I got one instead from advance which works great and was less than half the cost. Other than that, everything else I got new at the dealer.
Here is a direct link to the diagram:
JeepForum.com - Registration
The previous owner must have removed mine for some reason as mine never had one. I had to tighten the pulley by hand, which was the stupidest thing and couldn't understand why there wasn't a better way. Then I saw another Jeep owner remove their belt, and realized I was missing something. I can finally get proper tension on my belt, and it doesn't take me 20 minutes to do so anymore.
Edit: I have a 98 2.5l as well so yours should be similar.