Hey, I'm actually over in North Hollywood. Personally, I'd let it go. When the o2 sensor goes bad, the check engine light will come on. That's what happened with mine when I first bought it. It would come on and off, it failed the smog test because it was giving out faulty readings. As long as you're passing the smog tests and the check engine light is on, your O2 sensor should be fine.
When it DOES go, its a very simple procedure. Since it's a southern car, there shouldn't be much rust, worst case, spray a little WD-40 on it. But mine is a 97' with 70k and it took me all of 15 minutes to unscrew it and replace it.
As for your gas mileage, make sure you use Chevron, Shell, Mobil, etc. Stay away from Amco or those cheap ones, they have a lot of additives in the gas that leave deposits. I had the it professionally burned off of the heads and intake. You can try other things like changing the air filter, spraying out the throttle body, and maybe a bottle of injector cleaner, (I've heard good things about the Lucas Tune-up in a bottle)
Other things, if the wheels aren't balanced, that will cause a decrease in mileage. Same with the oil, if it gets sludgy, it will slow things down.
Currently: '14 JKU
Past vehicles: '92 YJ, '97 TJ, '10 JK