4 wheel drive won't work

There is a vacum powered actuator on the front differential housing on the passenger side. It gets it's vacum from two valves mounted on the firewall just behind the battery(to left). The white line feeds manifold vacum to the valves. Check for vacum at white line with engine running. If no vacum there is a break in line from manifold. If vacum check the pink and blue lines by applying vacum to each one. The vacum should hold. If not check lines from valves to vacum actuator. Most likely it will be the actuator. If both lines hold vacum check valves. Remove electrical connectors from both valves. Connect a wire from one of the terminals of the valve to ground. Touch a wire from the battery + to the other terminal of the valve. Listen for a clicking sound. If no sound valve is bad. Replace valves(s). If this doesn't lead you to the trouble it's probably a mechanical problem inside the differential. However most likely there would other noises.

Is the front axle turning, if it is there is a motor actuator on the front driveline it may be run from 12 volts, or vacuum. May be a fuse if there is no power, if there is power and or vacuum it may be the motor actuator that throws a fork-like thing to engage the driveline-axle. Probably you've heard reccomandations to occasionally use 4 wheel drive to keep parts from sticking. I'd say actuator is stuck.

Either your hubs aren't engaging(remove and clean and repack w/grease. or the front shaft or axle u-joints are bad