I was towing my boat, attempting to get on a 4 lane highway via on ramp. Truck refused to shift and went into limp mode as I was attempting to merge. Vehicles behind me on the ramp and coming up behind had to slam on their brakes. Diagnosed as a bad valve body. The issue is very widespread across GM vehicles, GM has a national backorder on the valve body as there are a ton of vehicles waiting for them. GM has not issued a recall at this time.