I think I may be having the same issue and I'm going to take a look at my fan clutch after work. I've got a 2004 Silverado 5.3L and I'm having warm air blow when I'm at idle and when I'm driving I have my A/C set to 60 degrees (lowest it'll go) and it's cold, but not nearly as cold as it used to be.
I bought a can of AC Pro last week and when I hooked it up and was checking the pressure I was getting pressures constantly in the red, but I'm not sure if my compressor was kicking on - which has to be on for an accurate reading correct?
Any suggestions?