Flushing the heater core seems to have done the trick. On my vehicle the blend door motor is not behind the glove box, but is near the floor just above the transmission tunnel. We could see the actuator turning when the temperature was adjusted. It was always possible to make the air colder. Sometimes the air would be warm and then go cold. The fact the air temp seemed under control led to the decision to flush the core. The flush pushed out a lot of crud.
For others reading this, here’s a link to further info on blend doors and their replacement:
http://www.heatertreater.net/Grand%20Cherokee%2093-98.htm
Note for 1998 especially, your blend door motor may be located either behind the glove box or just above the transmission tunnel on the passenger side.