You shouldn't have any problems. Use your Pac-brake going down all grades and if it is still picking up speed, just tap on the brake pedal for a couple of seconds. Do not ride the brakes going down hill.
I just got back from Yosemite, Ca and it was a piece of cake. Been to Yellowstone many times and I do not remember any steep grades, unless you come down through Cody, Wy. I believe it is highway 14 which is very steep according to posts I have read. I usually come down from the north coming from Montana, or the west coming from California and it is not bad at all.