No exhaust pings? http://www.gm-trucks.com/forums/topic/155069-pinging-popping-grinding-sound-from-rear-in-v4-mode/
My truck went in today for the 4HI issue yesterday still waiting to get it back.
Received a call last Thursday. A GM engineer called my local dealer(Chicago area) and said they are going to be mailing out a new part, I don't recall what the was called. Hope that this part works, stuck in 4hi currently.
It has to have the code cleared 1st. After that you have to remember to turn it back to 2wd, if you do not turn it back to 2wd the error will come back. Going in for the 2nd code clear tomorrow.