You've simply got a bad VC. The VC can fail in two ways, it either fails locked or it fails unlocked. The fluid can permanently lock the plates together or the plates in the VC can 'polish' so that the fluid wont lock them together. Its a good design, but its destined to fail. Just a question of when and how. Most people probably wont realize when they fail 'open.'
The 96+ 249 sends power to the rear until the rear slips (I've read a 10/90 split), then the VC starts locking up to send power to the front wheels. A pre 96 249 sends out a 50/50 split until either the front or rear starts slipping, then the VC starts locking the two together.
I have a 94 zj with a failed 'open' VC. Meaning just the rear or front can spin. I also have a 98 5.9 with a failing 'closed' vc. It doesn't chirp the tires but you can feel it binding at low speeds. Not sure if Im going to spend $800+ to fix the VC's. Most certainly NOT putting in a 242 or 231.