I have a 23' GMC 2500 AT4 with 34,000 miles on it and am having the same issue. The truck has occasionally had some hard downshifts at slow speed approaching red-lights and such since brand new. A few weeks ago while hauling my 43' 5th wheel, the truck attempted to down shift on the highway around 65mph for a slight incline and it was VIOLENT. Felt and sounded like the transmission just fell out of the truck. Made it to the lake and spent the weekend. A few days after being back home the truck stumbled trying to shift leaving a parking lot, went into limp mode and threw P0747.
Truck has now been at the dealership since 5/6 waiting on the backordered valve body with no ETA available, I was told to wait patiently and they will call when it's done. This is completely unacceptable for a $100k truck. I own a plumbing company and just recently decided to upgrade my fleet with some brand new GMC 2500 utility body trucks, which I am now regretting.
Making $1,650 payments on a truck that I may be without for months is ridiculous. My summer is now also shot as I have no way to haul my boat, or 5th wheel anywhere without purchasing another truck.
Seriously considering switching to Ford for my personal and fleet vehicles.