A/C Not Working Properly

[LS1_Unleashed]

I have a '04 Silverado 1500 with the digital dual zone a/c and for the first week I had the truck the a/c worked fine and had no complaints, recently though the a/c does not work properly. I changed the blower resistor to the newer style and it did nothing. When I turn it on it blows on the lowest setting and you can only really feel it when driving at speed, at idle you can feel any air coming out. The blend doors seem to work properly, but the a/c will not blow harder on any settings. I'm just trying to troubleshoot this truck without paying someone to do it just yet. I have thought it could be the blower motor, but why would it work only on the lowest setting if the motor is bad. Also, could it be the control unit itself? What are the other options of things it could be? I have a set of manifold a/c gauges so I will put those on and figure out if the pressure is low, but as I said, when driving at speed the a/c is ice cold but just barely blows, when at idle, it doesn't blow at all. I'm new to this truck so I'm trying to diagnose it as I go.

Thanks for any help you guys can provide me with!