AC only blows cold at max setting
 

It's hot in South Carolina. Boiling hot down here. I have a 91 Pickup 4x4 with the 3.0 and a 5spd. My AC only blows cold only if it it is in recirculating and max fan mode. If I drop it to any other setting, I lose the cold air. The compressor is new, and I just had the system professionally charged. Any ideas? Thanks.

Sounds like an air flow problem
 

Remove the front cowl replace the screens. Remove the dash, remove the airbox, clean out all the debris that fell through the holes and now plugs the air fins of the heat exchangers.

Wouldn't hurt to replace the seals/wipers when you rebuild the airbox.

Other causes for bad flow would be.. dieing fan, fan switch, or blower resistor.

O.P.,
Terrys87 has post on cleaning / taking the blower / heater core / evaporator box apart. (22RE)
That might help. IF you do it, pls see if possible to put a cleaning port to facilitate cleaning with out taking the assembly apart - maybe a hole big enough for a tiny vacuum crevice tool or flexible hose to insert in, and sealed with a plate and rubber/foam gasket.

I would bet the evaporator coil is covered with the years of dirt