I've yet to own a vehicle where I couldn't do my own oil changes. In the last twenty years, I've had my oil changed by a dealer once and they added way too mach oil, once by a Jiffy Lube and they busted the oil sensor above the filter when they changed it. I've done hundreds of oil changes over my lifetime since I worked in a garage for 2-1/2 years. Either way, I knew before I bought my last four or five cars if I could do the work myself.
I just trust anyone else anymore. Besides, quickly lube joints use shit for filters and shit for oil, at least the dealer used the correct oil for the vehicle!
Your tranny problem is nothing I can or would diagnose over the web. Too many issues and I've never been a fan of GM anything anyway. Now, they're dead to me as gubmint motors..