2001 jeep cherokee 4 wheel drive not engaging..?

I have a 2001 jeep cherokee straight 6 cyl and am suddenly having an issue with the 4 wheel drive. it seems to go into part time and the part time light lights up.. But I'm not getting anything for full time 4 or 4 low. Any help would be great..|||jeep computers are kind goofy like that it wont make the complete switch over to full time until it deems necessary to maintain traction. its been a common occurrence since 84.