service engine light/misfiring

You should expect to pay for an hour's diagnostic time, I think. Considering the range of possibilities that could cause the light to come on, I'd say a professional diagnosis is going to be cheaper than throwing parts at the car and trying to get lucky. Without thoughtful diagnosis, it's nearly impossible to fix a modern vehicle. If you want to see a specialist, here's a directory link for you: https://repairpal.com/gmc-repair-in-memphis-michigan