You guys are giving me flashbacks to my 2008 avalanche. I chased this EXACT same issue for months. Ended up being the 15 pin connector to the transmission, or the Transfer case control module. I disassembled and cleaned those 3 connectors and the problem went away. This was after replacing the module above the spare tire, and the mega fuse and all that crap. There was a TSB out that was 120 pages long dealing with this issue. Loss of GM Lan.