Heater will not switch from the defrost to the floor on a 2004 envoy

Sounds like the diverter door is bound up causing the motor to bog down under load. You'll need a manual for that job. It's a pain in the rear to get to.

it is a leak in your vacuum tree that controls the blend door. You can buy spare vacuum line at a parts store pretty cheap. $1 per foot or similar. the trick is finding the bad vacuum line. look for small plastic line coming out of the firewall and replace if cracked

Do you hear a clicking noise when you turn your truck on? Its probably the actuator. While the defroster was on I removed the fuse locted under the rear driver side seat. I think its 36B. It stopped the clicking noise and my air vents work well now.

Try recalibrating the HVAC system first. Set all HVAC controls and switches to their OFF or CCW positions, then disconnect battery for a couple of minutes, reconnect battery and don't touch any of the HVAC controls for 2 - 3 minutes. If this does not correct the problem, you may have a stuck blend door or another bad actuator.

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

u have to take your dashboard at the glove compartment to cheack the vacuumpod ot your damper door

Check for a vacum leak from the engine to the a/c.The color of the hose is black. Also check if the heater control valve hose is unplugged. If you have vacuum the it sounds like the door for the vent/floor/defrost is broken.

probably is a small seep in the heater core. does not have to be leaking enough to find any inside the car to have a strong smell in there just a small seep that makes the heater core wet will do it.

you have a bad control switch.