The 4x4 vacuum system only operates for about 1 minute after moving the dash switch then it shuts off. If the hubs are leaking the truck will switch to defrost when the dash switch is moved and then after a minute or two the vents will go back to normal. It is a common misconception that there is always vacuum at the hubs when in fact the vacuum is only a pulse that latches and unlatches the hubs and last less than a minute and is either 6” or 15” depending on whether your locking in or unlocking. There is a vacuum solenoid/regulator on the passenger fender that controls the 4x4, I suppose it could be bad and causing the hubs to always have vacuum but then they would not lock or unlock properly. I have the wire unplugged to mine so I can use 4x4 Low to back trailers without locking the hubs so I can turn sharp.