problem has occured for the past 2 weeks.
the air or heat only blows out of the defrost no matter what control is set on. on 2004 GMC Yukon Denali
You have lost your vacuum to the control head. You most likely have a bad control head. Defrost is controlled by vacuum not a blend door.
all three are good answers
Vacuum leak under hood pass. side near firewall.
maybe your blend door is broken