you could clean your idle air control on the throttle body it take torq head sockets, and the throttle body could need cleaned as well, carb and choke cleaner will do wanders on both.
Your '90 4.0L is fuel injected and it's pretty easy to dive in and remove the throttle body. I'd take it off and give it a good bench cleaning with carb cleaner and an old tooth brush.
The throttle body on the left is what's on your engine, the gold looking thing is the IAC (idle air control) that adjusts your idle per the computer's instructions. The TPS (throttle position sensor) is directly to the right of the IAC. Keep in mind that if you remove the TPS there are very specific instructions on adjusting it once it's reinstalled.