We basically did the reverse trip in 2010.
US2 all the way! Really only one highway to remember
Through Kalispell, Libby, Bonners Ferry, Sand Point, Priest River, Newport, Coulee City, Leavenworth . . . will get you on I-90 for just a couple of miles in the Spokane area.
Beautiful trip. Great scenery. Light traffic. Decent roads. Did I mention just a couple of miles of interstate?