An Autism Bulletin reader shared this useful tidbit: The National Park Service and other federal agencies that manage recreation lands around the nation waive admissions fees for citizens with disabilities and their caregivers and parents.
What you need to gain free admission is an "Access Pass," explained here at a website of the U.S. Geological Survey, which says:
The pass is for citizens or permanent residents of the United States, regardless of age, who have been medically determined to have a permanent disability. It provides access to, and use of, any Federal recreation site that charges an Entrance or Standard Amenity Fee and provides a discount on some Expanded Amenity Fees. The pass must be obtained in person.
The website goes on to explain that documentation is required to demonstrate a disability that represents "a permanent physical, mental, or sensory impairment that substantially limits one or more major life activities, such as caring for oneself, performing manual tasks, walking, seeing, hearing, speaking, breathing, learning, and working."
Once acquired, the Access Pass is good for life. The website link above has more details and information about acquiring this pass. Fees may be more complicated than this website suggests (for example, you still have to pay parking fees at Mount Rushmore). But it will allow you to avoid many fees, including those at Yellowstone.