I recently purchased a 2005 F250 super duty and the four wheel drive seems to work intermitantly. if i lock the hubs it works, I turned them to automatic and it worked for a while. Now im not too sure how this system works, but I know that that it applies vacum in both steering knuckles when you select 4x4. When I select four wheel drive on the dash within 10-15 seconds my heater goes to defrost and remains there until I put it back into 2wd. Hence a large vacum leak. Do the automatic hubs operate on vacum? or is it something different? if it is what does it actuate?