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In late May it would be extremely unlikely to see any winter weather issues on the Interstates anywhere in the country.

Keep in mind, if you drive directly from Vegas to Grand Forks, it would take you at least 3 days, so if you spend "a few" days in South Dakota, you won't have a ton of time left, but there are no shortage of things you could stop and see. Zion, Bryce Canyon, Arches, Canyonlands, and Rocky Mountain National Parks are all very close to the direct path. There are also other parks, like Grand Canyon, Monument Valley, and Mesa Verde, that aren't on the direct path, but could be worked into your plans.