The IAC valve is PWM controlled, kinda like a servo. Depending on the duty cycle the pintle will move more or less.
I still think you might have a vacuum leak but since I've never seen the 3.0 engine I can't tell you what else to look. I'm sorry.
Have you tried a propane torch to find leaks? What happens if you block the air tube while the engine is running. Will it die instantly or still keeps idling?
Oscar.