Yellowstone has a very broad range of temperature changes throughout the park. In higher elevations, you can expect temperatures to drop to below freezing at night. Even in early Summer, when you can expect temperatures in the 70's, I have experienced cold, inclement weather! I have always taken a light jacket with me, even in the warmest summer months.
If you would like to see National Park Service suggestions for Yellowstone's seasons, click here.
According to the Skywalk operators, which are not part of the National Parks Service, it is not necassary to book your tickets in advance. Busy season in the Grand Canyon is not until summertime, so it is not likely you would be running into any crowds either way.
Enjoy your trip to the Grand Canyon, it is a view you will never forget!