Need help re. Jeep catalytic converter...

I know there are Jeep forums, but I don't want to go creating another account somewhere else, plus I know all of you have tons of knowledge...so here I am. Here's my issue: My 1996 Jeep Cherokee (4.0, in l6) has been getting absolutely HORRIBLE mileage lately. As soon as I hit 160 miles, the light comes on (when I refill, I put about 15 gallons in). Roughly 10 mpg on average. I have replaced spark plugs, air filter, fuel filter, and just upgraded my fuel injectors today with the Neon ones (4 ports instead of the stock 1 port). I just filled up, so I'll see if it adds to my mileage, but I want to be prepared. So here's where I need help.

a. What else can you think of that might be causing such horrible mileage? I used to get around 300 miles to the tank, on average.

b. I am stationed in Delaware, but my Jeep is a California original vehicle, with a CA emissions catalytic converter. I haven't gotten it pressure tested yet, but if it is bad, and I were to put a non-legal CA cat on it, with the 02 sensor port, would it throw a code? Would it negatively affect performance? And what if I were to get stationed in CA again, would they make me change it, even though I'm military?

Thanks ahead of time...sorry for the novel. It's driving me crazy putting so much gas in this thing!