01 O2 Sensor Question

So I've read a lot about o2 sensors both in posts on here and in my factory service manual. I understand that the 01 XJ's were equipped with four of them. Two upstream in the exhaust manifold i believe and 2 down stream. I've read the second downstream one is there only to confirm that the cat is doing its job. My question lies with the replacement of them. I currently am pushing about 113K on my Jeep and to my knowledge the sensors have never been changed. Lately my gas mileage has taken a dive. It has new plugs and the cat is fine. The O2 sensors are the next logical potential culprit in my mind (correct me if I'm wrong here). So finally to my question. Is it necessary for me to replace all the sensors or only certain ones.

Thanks.

your mileage will be affected most by the upstream ones. If no "MIL" or codes are present, but your mileage is worse that before, I'd suspect higher alcohol content gas or slow O2 sensors.

I agree with shorty--the engine uses the upstream sensor to determine how much fuel to use, and the downstream one for emissions data, as you've said. I don't think changing the post-catalytic convertor sensor would affect gas consumption at all. Earlier this winter I got a '98 XJ (155K) with probably the original O2 sensor and it ran so poorly I thought there was a transmission problem--it would hesitate and buck like mad when I was driving it cold (possibly the sensor heater element no longer functioning since it would drive fine when it warmed up). I replaced the O2 sensor at someone's suggestion here and it runs great now. My 98 only has two O2 sensors. My 04 Subaru only has two as well. Not sure if there would be a reason to have a completely redundant set--you might want to double check that.