[old_master]

Sounds like you're on the right track...low vacuum to the actuator. There is a vacuum switch on the transfer case with 3 ports: One goes to the actuator under the battery. One vents to atmosphere and is usually attached to the transmission dipstick tube and has a small filter on the end of it. The third goes to a vacuum reservoir under the hood, and has a "Tee" and check valve then goes to intake manifold vacuum. Some systems use an electric solenoid, in which case it is mounted to the firewall, a little right of center. The vacuum switch on the transfer case can also develop a leak. Hope this helps.