My BIL has a 4L auto, it had a few problems that I diagnosed a while back, do a diagnostic check on it, you don't need to use an OBD scanner.
To do it turn the ignition from 'OFF' to 'ON' 5 times, on the 5th time the diagnostic system should start flashing out codes via the engine light.
Just type any codes you get into google (jeep code 5 etc), there was a website that had all the details but it has been that long that I cannot remember.
EDIT: Try this - http://www.allpar.com/fix/80s-codes.html