on the way out stay south and avoid Chicago at all costs.... don't use I90 through chicago... stay on the outside loops at a minimum
10-12 hours a day driving to get to SD in 3 days 1800miles/3=600 miles a day...you have to stop at wall drug though ;o)
on the way back once you leave the mountains you are going to drive across allot of nothing no matter the route... 25 is an okay drive.. the shorter route will be less painful