My Defrost doesn't work

SOURCE: Heat wont blow through heater or defroster vents

The door in the heater box is most likely stuck. It is located under the dash on the passenger side

SOURCE: Air conditioner blows out vents and defrost vents

sound like cotrol not swapping them over

SOURCE: Heat wont blow through heater or defroster vents

common problem, check under dash by the gas pedal if control arms are muving went selecting from ac to heat cuz this it is the control for the vents to go from ac to heat and to defrost.

SOURCE: my 2004 gmc envoy loses engine pressure when i

Don't know what you mean by engine losing pressure, but as far as the air not coming out of the correct vents, try the following;
Pull the HVAC-B (#36) fuse for 30 seconds (located in rear fuse panel, under seat), then put it back in. Then turn the ignition switch to RUN (no need to start the engine). Don't touch ANY controls for 90 seconds. I don't think it matters what position the controls are in, but some people think it works better if you have the HVAC system turned off. After 90 seconds, the actuators have gone through their paces, the controller has new memory data on the end stops of the rotation of each actuator, and you might be good to go