2002 Hd Hvac Is Dying

My beloved 2002 2500 HD Crew Cab 4x4 is going back to the dealer tomorrow. The AC quit with NO sign of distress or warning a few months back (it also needed tranny reprogramming to fix a nice "lurch" it developed). They took it apart TWICE and finally got it working. This time, the heater died (yesterday was colder than -10 F) and I am not hopeful they will get it right the first time. Any ideas on what could be causing the death of my HVAC? I kind of wonder if the tranny problem is related to the HVAC since some systems up the tranny pressure/shift points to compensate for the A/C compressor. Any help is very much appreciated.