I have the same issue that the OP posted, Service ESC and Service steering system, my truck has been at the dealer for 6 weeks waiting for a steering rack that is on national backorder. same thing, parked the truck, came out to no steering, 90 min later when the tow truck came it worked fine. ive been racking the miles up on a GM paid loaner. GM hasn't gotten back to me yet on a timeline when the parts are going to be available.