The pre '00 Cherokee had all the climate doors operated by vacuum. '00 and '01s used a combination of vacuum and electric actuators for the climate doors. There is a good chance that the blend door is either sticking or not working. Switch between heat and A/C and you should here the blend door operate. If not, then it's the likely culprit. The blend door is behind the glove box. To repair the blend door, it requires dis-assembly of the dash.