o2 replacement-- do I need to replace both?

My Jeep in running poorly and getting poor gas mileage so I want to replace the O2 sensors. They are way over due for a change. Do I need to change both, or is one more important than the other?

I would replace both, do a tune up with factory parts including plugs, cap, rotor and wires, new air filter. Before doing anything though use a good injector cleaner and you might want to pick up a can of Mopar top engine cleaner. It is not a pour in like the GM but is instead a spray, costs about $4 a can. On the O2 sensors be sure to use an O2 sensor approved antisieze compound if one is already not on there.
Be careful when using the spray type carb/fuel injector cleaners on the throttel body. Most of those cleaners are not plastic friendly so keep the stuff away from the TPS and air flow sensor or better yet take them off before cleaning the throttle body.
Another reason for running poorly is the cat, it could be plugging up or plugged up and I don't know of any way to test it short of disconnecting the header and blowing air thru it.

IMHO replacing the top one (pre-cat) would be most logical since it's data is used in the fuel calculations - the post-cat sensor primarly monitors the effecacy of the cat and the rest of the emissions package -- based on your desire to change the O2 sensors and your reported complaints, this assumens that you've cleaned your fuel delivery system, verified it's operating pressure and eliminated the general "tune up" as causes...