GM-GMC: 2003 grounding issues, 2003 gmc 2500hd, electrical expert

Question
I just brought my truck in and I was told that there are many codes in fault for my truck there are some to do with ABS, some engine codes, I was told that all of the components share the same grounding points in the truck and it is more than likely a grounding issue my truck is a 2003 gmc 2500HD 4X4. The reason I brought the truck in was periodically the truck will shift from 2 high to 4 wheel drive by its self on the indicating lights but the truck has not actually gone into 4x4, if it decides 4x4 low than it changes the truck to low range but dose not put the tires into 4x4 low. Do you think this could be a grounding issue or something else.

Answer
are there any aftermarket alarms installed or were installed? sounds to me like the wiring is croseed. and could be a grouding issue .. i am not a electrical expert but since its throwing codes sounds like wires were tampered with or the ecm is going bad.