Check under the hood of the car on the side opposite of the battery for the vacuum actuator. It's by the break master cylinder. Look at the hoses for cracks and also check down at the front suspension out to the wheel hub for cracks in the hoses there. The internal hubs are a dual diaphragm design and do get contamination in them that will not allow them to seal to activate because there is not enough vacuum. They will also not deactivate if there is not enough pressure.

The best solution is to switch to the free wheeling hubs. They are much more reliable and are stronger too.

Logansportage