It could be (it could be a lot of things), but I would check the cold-start injector first. I assume you're not in Southern California, so temp is the issue. You can pull the electrical plug off the injector, if your problem doesn't change, you may be on the right track (of course, that doesn't tell you if it's the injector that's bad, or the ECT providing the signal, so the next step involves a voltmeter on the connector).
The ECT is located in a fist-sized aluminum block on the very back of the engine, below the top of the intake manifold, and has a lot of hoses to it. Work on any of those sensors is difficult.
Last, just about anything can cause poor mileage. Have you checked your timing? Tested the TPS (throttle position sensor)? All easier than the ECT.
1994 6cyl 4wd XCab -- all stock