Idle Air Controller is a small motor mounted on the side of the throttle body. It operates a small plunger that extends in and contracts out of a small hole in the TB to allow a change in air flow altering the idle speed.
One problem is carbon build up. It would help to pull the TB and give it a good bath with some B-12. Remove the IAC and be sure to clean out the chamber that the plunger goes in to.
Good chance the IAC is bad and needs to be replaced as well.
Seen this on a couple different Jeeps of mine, usually the older ones, early to mid 90's. Engine will idle super high and seems to be taking off, but once you get it to a half-assed idle and try to frive it, it will idle down so much it tries to stall at a stop.