2003 Jeep Cherokee problems unlocking the differential

Ok so if I understand right your front is running at a different speed then the rear? I have to ask a few questions ok. Any new tires on this Jeep? Has there been any diff work on it? Have you checked to see if the front and the rear are the same ratio? Get back to me with what you can.

Ok if you would mark all 4 tires and then roll the car straight for one rev. Of the tires and see if any of the tires are not rolling the same distance. If a tire is really low it won't roll the same distance either. See the rear can't lock that way as the rear end is a limited slip rear diff. Meaning that it has clutches. Now if the rear end fluid was changed and the limited slip additive was not added then that could be a problem. Any way check the rolling distance first and let me know how that goes.