suburban a/c problems

My 99 suburban rear a/c works great , but my front a/c quit working on a trip last month , The a/c light is on , the blower works fine , all the vents are blowing lots of air , I checked all of the blend door motors and they are all working when the dash control is moved into different positions . the only time it even gets kind of cold up front is when the rear air is on and the veichle is traveling on the freeway . When I turn off the rear air , start the engine,turn on the front air the a/c compressor kicks on and cycles as usual but no cold air is blowing from the front vents ,only warm air . Any idea what is causing this ? The only thing I could think of is a blend door has came loose in the box .I felt the a/c lines that go thru the firewall and the hose is cold going into the vehicle . Any ideas on whats wrong ????