Thermostats fail in the open position, that way they don't cook the engine if they fail. I'm betting it's either you need a new radiator or you need to add coolant. Maybe you might try flushing the system and adding fresh coolant (mixed 50/50 coolant and water), and make sure the pressure cap on the radiator is good.
Yeah, did yesterday. I bought a ranger hoping to get decent gas milage, and I get like 15-17mpg with my 4.0, hardly better then a 5.0 v-8. I put in $40 yesterday, not even 3/4 of a tank...