Yes, are you following the manufacturers guidance for flat towing? It sounds like the computer is getting false feedback while you are towing. My RAM 3500 4x4 has a procedure to put the transfer case in neutral, then I put the shift back in to park and turn the key off and remove it. Not sure about GMC.
Ron & Peri
2007 Travel Supreme Select Limited 45SS24
Cummins 600hp ISX15 Spartan K3 Chassis