I have a stock 4WD '98 with a 5 speed stick, I tend to get about 26 on the highway when its flat and on very gradual hills. Keeping it under 75 is key though, at like 75-80 its really drops off. I also have a 4WD '00 XJ with an auto and manage about 20 or so. In the city, I get about 16-17 with both. If your looking for an early 90's XJ, watch out for the carbs, I had a late 80's XJ fall due to the bad fuel system. The later models have electronic fue injection and are extremely reliable aswell as efficent. I've used, 2WD autos and manuals and they tend to get a tiny bit better milage, nothing really worth fretting about. Plus, if it 'aint 4WD, is it really a Jeep??
Of course it is, but you get ym point.