Tune up first. Champion plugs, the best wires you can find/afford. BG44k in the tank, new fuel filter, Mopar Decarb Spray (avail at dealer) spray can thru throttle body at idle, turn off, sit for 15 minutes, start engine, smokes like a freight train and cleans all the valve, pistons, chambers, etc:, good stuff..., make sure tranny is in good shape (no slipping), fresh fluid, etc: synthetic fluids front to rear (diffs). tire pressure set to proper levels. change oil and use something like 5w-30 or 0w-40 synthetic..
Maybe have a lazy O2 sensor, this can be verified with a scanner or volt meter.. For the price of a O2 sensor I would check it before replaceing it. If it is lazy then yes replace the O2 sensor..
Also check for plugged exhaust system, If the system is plugging up then milage will go to crap and nothing stated sofar will change anything.
just suggestions... all i can think of right now..