defroster

i need help my defroster stays on defrost except when i change from 2x4 to 4x4 or the other way around it goes to vent for a short time but then changes back in 30 secs to a minute any one have ideas for me its a 99 f250 sd diesel

The 'default' position for the climate control venting is defrost.. If you have a vaccum leak the vents will blow from the defrost position.. Check to make sure that the vacuum pump on the R/passenger inner fender runs/pulls a vacuum, (but not a continuous run) and that the hoses at the pump/reservoir and running down to the vacuum hubs aren't leaking and still connected.. As well as vacuum lines ect. under dash.. Vacumm seals in the hubs are know to leak as well.. If the electric vacuum pump and hose prove out as well as vaccum hoses routed under dash and no leaks are detected at the heater plenum, try and disconnect/plug the vacuum 4x4 hub hoses at the hubs, or at the upper connections in the inner fender and see if the venting returns to normal.. If so,, connect on hub line at a time to check for a leaking seal R to L... The hub seals can/should be checked with a vacuum pump for seal if a leak excists.....