I believe you have a vacuum disconnect system in the front axle. If you look on the front axle on the passengers side, you should see a vacuum line connect point. With it disconnected, you will not be able to engage 4x4. And, there is a point where once engaged, it will not disengage as long as there is vacuum present. As I recall, when you disengage 4x4, you lose the vacuum. The vacuum operates a stepper motor that slides a sleeve which locks the front axle together.

Slow at disengaging indicates either the vacuum port is plugged or the sleeve has a binding that is not letting it retract.