From Detroit it seems the long way around to use I-70 all the way from the Midwest. I'd suggest I-94 to I-80 near Chicago (try to be going around Chicago on a Saturday or midday - worst time would be Friday between 3 and 8 PM.) Stick with 80 all the way west to I-76, which cuts off toward Denver near Ogallala, Nebraska, as 4runnerguy suggested. I-80 west of there will put you through all the SLC traffic (yuck) and as long as you're going to St. George, I-70 will put you into Utah way south of SLC and well on your way to SG. Take I-15 south from I-70 to St. George.