I am also having similar issues. First thing i'm going to do is look in my Haynes manual and find the vacuum lines that feed the HVAC system. While the truck is running i'm going to look for air leaks, which will sound like air being sucked into the vacuum lines. If i find any, i'll fix them. I'll also check any components along the way, like check valves. I really don't feel like ripping my dash out, so i hope any leaks are outside the dash.
I currently have the same issue and have for a while, what I do is turn the fan dial all the way to off, turn the selector to vent then crank the fan to HI. Then to put it back to defrost I do the same thing, after I get it where I want it I dont touch it till summer time or winter time (only do it twice a yr) just ganna keep doin it till I round up the cash to get it FIXED, grom what other guys are sayin it costs around $800 to get er done, have to tear the dash apart