Stumped!

I have a '98 GMC Safari with front and rear AC. Over the course of just a day or two, the front system started blowing ambient temperature air, but the rear system is still working fine. I verified that the temperature door is closing off warm air from the heater completely, and the front fan is working fine. The rear has a separate orfice tube, evaporator and fan, but the compressor and condenser are shared between front and rear. Suspecting a plugged orfice tube or hose, I replaced the front orfice tube and hose assembly, which is a unit with includes the suction line from both front and rear evaporators and the line between the compressor and condenser. The orfice tube had no obvious obstructions. I also replaced the accumulator, and evacuated and recharged the system with R134A. The problem still exists. The clutch is cycling within specs, switching off at 21 PSI and on at 45. The compressor is controlled by a pressure switch on the accumulator. High side pressure is 145 - 160 PSI with a 65 degree ambient. The outlet of the orfice is around freezing, but the evaporator exit tube is room temperature. I don't see how the compressor or condenser could be involved, and the only other thing I can think of is a restricted evaporator, but I think this would be rare. This one has me stumped, but I only work on AC systems whenever my personal automobiles have problems, so I am definitely a novice at this. Any advice would be appreciated before I discharge the system again.