[hundahunta]

If i's not mistaken, this is a vacuum locked front diff. If you look underneath the vehicle in the front diff you will see 2 vaccum lines and a connector. The connected has a switch that tells when you are in 4 wheel or not, try and jump the switch and check you 4whl light. you need to check vacuum at the front whheel drive actuator when shifted into 4 wheel. 1 side will be vaccum in 4 wheel the other will be vacuum when shifted into 2 wheel drive to move the locking gear back and forth inside of the diff. I would also check the main vaccum line going to the vacuum switching valve on the transfer case and in the engine comparment