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I'm not sure why you need an open air jeep tour in Custer. Will you be in a rental or without vehicle? The buffalo in Custer are the most easily accessible I know of. The park just isn't that big and I have traveled off road but most of the buffalo move continually so its not hard to photograph them close up.
I've been to Custer State park countless time, always stop on my annual trip out to Wyoming/Montana. The center of the park has dirt roads but unless you hit something unusual a car will easily provide all the access you would need. If you are going in August it will be dry more than likely.
Get into the park early and you'll have the wildlife to yourself, by early I mean before sunrise.