What that means is you have a leak in your hub lock vacuum system. If the system loses vacuum, the HVAC defaults to defrost position. Could be the hublock solenoid valve or the hubs themselves. If the defrost condition is just temporary as you switch to 4x4 then the solenoid is probably OK, letting the vacuum build back to normal after the vacuum pulse to the hubs (to lock or unlock) is finished.

 

Makes perfect sense - been having sporadic problems getting the hubs to lock..sometimes they...will other times I got to keep switching back and forth to get them to lock.

Any idea where to check first for the hub lock vacuum leak?

 

The black rubber hoses at the steering knuckles fall apart after about 6 to 8 years. If yours are OEM, they are probably shot. A warning too, if they are original and you grab them, your hands will be black!

If they look good, start following those lines back up into the engine bay looking for cracks and breaks. Its also possible that the seal is bad in the unit bearing.

 

Just saw the "STICKY" on ESOF and I'll take a look at that too - thanks!