yup, I am going to say IAC as well. had mine go on me last year, same symtoms, even blew some dark smoke out the exhaust once with it that way. it wasnt expensive at all from advance auto, and only took a few minutes to replace, and after that, you gotta drive the truck for a bit to "teach" it to work right for you.

1991 Rodeo XS
3.1 V6 4x4
136,000 + miles.