My Yukon Denali has been in service 3 times for the "service 4wd" issue and still no fix. What the dealer told me is that the problem is coming from starting or turning off your car while in the "auto" position. He said you can switch to auto after you start as long as you switch back to 2 wheel before turning off. The system was obviously not intended to work that way and he said they are working on a fix. That being said he told me they would have a fix in a week and that was now almost three weeks ago.
For me, the best resolution for all the problems I was having with my Denali was to trade it in , which I did this week. Between the service 4wd issue, the 8spd transmission now standard in the same model year, the clunky drivers seat, poor radio reception and a rattling exhaust it made the most sense to unload it now and minimize my loss. They way I looked at it was to sell now while new inventory is still low and used inventory is almost none and demand is high. They gave me $64k on trade and I paid a little over $68k with a MSRP of $74k. It cost me $4k to drive it 6500 miles which isn't to bad on higher end vehicles. I did consider going the "lemon law" route which I believe I would have won with the documentation that I had but it wouldn't have been worth the trouble because I would have lost the sales tax savings on the trade in.
Overall I can say that owning my 2015 Yukon Denali was a very disappointing experience and it is obvious GM has a long way to go to improve the quality of their high end suv's. I have had a few Denalis in the past and had zero issues with them and will probably still buy a 2016 2wd, 8spd trans. Yukon Denali because I love the new look and I need the space they offer. I also should add that my wife has a 2wd white diamond yukon Denali and she loves it!