ah ... well I'm sure the 70-200 f/2.8 II with the 2x III will serve you well in yellowstone. that's exactly what I'm bringing down to Ecuador this year. was debating renting a 500 f/4 but I'm not sure about the cost or the weight of hauling it around.
70-200mm with 2X is not long enough for wildlife in Yellowstone unless you are photographing the elk on the lawns, or buffalo that come up close. Even 100-200 yards away is a long ways.
Here is my 70-200mm with 1.4X and the animal looked to be quite close.
On the other hand, a 70-200mm zoom isn't the best choice at Old Faithful.
70mm backed off a long distance such that it was difficult to keep the crowd out of the image.