Mine is an 02 and here is the AC Delco part number for the door actuator ACDELCO Part # 1572684 . As far as the reservoir goes, mine is the same as yours and the nipple on mine leaked. Didn't know how to replace it so I went to the junkyard and got the generation 1 vacuum ball and am now using it.
To me, it looks like a real bear to replace and this only cost me $10. Works just like the fender reservoir does.
I now have the dreaded problem you have with most of the air coming out of the front vents with little on the defrost and floor vents. I replaced all the hose, 4wd actuator, transfer case switch, one way valve, and the HVAC Control in the Dash over a period of years to fix it. Problem was fixed then about 2 months ago this problem arose. Mine could be either the actuator I spoke of above or the ROCKAUTO HVAC control for the dash went bad after 3 years.
When it warms up here and I have time I will troubleshoot this. At least, with the cold weather we are having, I have hot air and can stay warm..