Heath, what are the exact dates that you'll be in the 2 Parks? Also, where will you be hiking while in YNP, and also in GNP? You see, the central plateau of YNP is 2000' higher than the northern part of the Park. Big difference in temperatures. Keep in mind that you could encounter a thumping from snowfall anytime after Labor Day, in either Park (or not!). It seems like your base layers and MH parka should work, especially if you wore a WP/B shell over it. I personally would carry a pair of puffy pants ((BPL Cocoons), with a pair of GL Reeds to protect them and add some more warmth. Good gloves and hat, too. The sleeping bag should be rated to maybe +15*F.
Clayton M., Mike M., and Dave C. should show up here soon, and they'll have some good advice for you.