does it blow through the defrost vents full time no matter what?
If so then you need to start tracing lines from your vacuum pump out and find where you are losing vacuum. The other thing is the vacuum pump itself, is it working properly? is your truck equiped with 4x4? Are your hubs vacuum powered? or were they and some one swaped it to something else and did not properly cap the vacuum line? I did have one time that my ac did not want to switch back after using 4x4, I ended up squirting some light oil down the vacuum lines to lube the different orings in the system. seemed to help after a little while. I haven't had a full time defrost blowing issue like yours but these are the places I would start.
Joe