Jim Dawson

I had what sounds like a similar issue with my 2016. Codes indicated a bad injector which I replaced with some money from GM's "we feel bad about this" fund. Problem gone

I think there is now some recall on this issue. Not sure if its for the vacuum pump, but when it happens to me, its always at low speed/rpms. I put the truck in Neutral, and rev the engine and pump assist kicks in to stop the vehicle. Not an elegant fix, but it doesn't cost me anything.

Re: Service 4WD Message and truck stuck in 4WD: This is a frequent issue which my local dealership didn't see listed for my 2016 Sierra Denali (6.2l). Service department says I'll have to leave the truck with them all day. The aforementioned TSB describes my issues perfectly, so I get suspicious and go to my local garage with a chocolate milkshake for buddy who comes out to clear the codes. Well doesn't my truck immediately shift into 2WD, issue solved (unless I want 4wd auto, which I never will again). GM should include my truck on the TSB in my mind. Apparently the issue is software and requires TCCM reprogramming to permanently fix it. I'm just glad I didn't get ripped for 6 or more hours of fruitless searching at my Lindsay ON dealership.