I would suggest pulling off the locking hubs and oil them. It only takes a pair of needle nosed pliers. There is a simple spring ring that retains them. Be careful not to mess up the o-ring on the hub. I just turn them on their ends and fill them with engine oil. Then work the manual lock a few times, and get it working more freely. Drain and then reinstall. It seems to make them more smooth. The hubs work on vacuum. A vacuum is pulled on the hub cavity, and atmospheric pressure pushes a plunger in the hub and forces several splined pieces to mesh, locking the hub. If you lose vacuum, (or shut off the 4x4) a return spring pushes the plunger back. If the splines bind due to torque, it might get stuck on. A benefit to oiling the hubs is that it will help seal any air gaps that might exist in the hub/plunger system.
eli benson