If you have a 4wd then there are issues with the check valve in the transfercase causing fluid to enter the vacuum line and disrupt the vacuum operated HVAC controls. If this is the case, you will have to replace the vacuum harness, vacuum operated actuators and HVAC harness plus the check valve in the t-case. So if this might be your problem, removing the HVAC controls from the dash and disconnect the vacuum connection on the backside. If you feel or see anything oily then you know what you need to do.