It is the typical recommendation that you replace your oxygen sensor(s) after 100,000 miles. If I'm not mistaken yours has 2...one just in front of the Cat convertor, and one behind. The system uses two as a way to determine if the cat has gone bad or not. I don't think they are too difficult to replace, I intend on replacing mine in the next few weeks, so I'll let you know. I found them for around 50 bucks each at the local auto parts shop.
1993 ZJ Limited 5.2 - no mods - Red with Gold (what were they thinking) trim.