Had the Yukon XL towed to the dealer.
Was on the highway about 50kms from home, on our way back from a day skiing, when all the warning lights went off and the truck started running like sh!t. Got the "Reduced Engine Power" message and it was a struggle to maintain 60km/h. Pulled over, took a quick look under the hood, then restarted it and all was well again... for another 3kms, then same thing. Found a parking lot to pull into, then called GM roadside assistance and waited an hour for the tow truck to show up.
It's at the dealer now, but I'm not too worried: I still have a letter GM sent me about six months ago advising of a known problem with the throttle position sensor. Symptoms are the same as mine, so I'm expecting it will be a quick, easy (and free) diagnosis and fix. Fingers crossed.