I have a 94 Trooper and I don't get gas mileage that is nearly that bad. I'd change the o2 sensor, unless you know that it was changed recently. The factory interval is 90k miles. I noticed that my gas mileage went up about 1mpg when I changed mine - and I didn't have horrible gas mileage. Obviously how you drive can effect it as well. Try driving as calm as you can for a tank and see how far you get. I'm not sure that I could make the low gas like come on in 150 miles, no matter how wild I drove mine though. I'm closer to 300 before the light comes on in mine. I guess that's about 17-18mpg, and I spend half of my time at 35mph, half at 80mph.
Some BG injector cleaner wouldn't hurt either. While you are in maint mode, the rear diff fluid probably hasn't been changed in a long time, and you could check the brake fluid and power steering fluid as well.
Other things to check: you are firing on all cyls? The 92-95.5 3.2 engines seem to have a habbit of causing their spark plug wires arc through the insulation after some time, but this is usually noticable in the way the engine runs.
You don't have an OBDII connector on your 94. You have a connecter near the console that will let you view any stored engine trouble codes. You connect pins one and three together with a short piece of wire. You'll find more than one connecter under there, but the one you want only has 3 pins, and it isn't connected to anything.