Update on pricing (Thank you!)
I took everyone's advice. Woke up early and called dealerships all over the country that were selling a lot of GMC's. They too, confirmed that the price was WAAAAY high and that gave me confidence that I could indeed find what I was looking for at a better price. I also called Costco and asked about what my options were with this dealership who was selectively honoring their program.
Once I took the emotion out of it and was OK with walking, I got a phone call from my dealer. We talked it over and I told him I was going to go elsewhere. Literally right before we hung up the phone, my dealer told me he'd try to honor the Costco pricing (supplier pricing). Just got a text that my price is now 71624.20 with $600 Costco gift card. That's down from MSRP on my vehicle, which was 77035.
I know it's not as low as Laura but now I don't have to hassle getting out there and getting the car back here (Utah) and the Denali I'm getting is the one I ordered specifically with what I wanted.
Thanks guys! You gave me the confidence needed to stand strong and get a better deal! Really appreciate it and now I don't have a pit in my stomach.