My personal opinion is that U.S. 2 was fine from Columbia Falls into West Glacier. NT routes you on a beautiful back road that turns into a rough gravel road. No thanks, that part needs at least front suspension.
When did you ride that? We (April and myself) rode that part of the NT/Great Parks North last summer (July) and the unpaved section was fine with our bikes. It was mostly hard packed dirt. Neither of us had front suspension, and also had front bags. We were running 32mm/35mm tires. April didn't mind that section, and she HATES gravel.
My daughter and I hit that around the middle of July, 2011. Took the back road up to Glacier, and then came back town 2. As I recall, there was about a mile and a half of two lane road near the bridge on 2, and we had no problem with traffic all the way into Columbia Falls. The washboard we hit was noticeable all along the unpaved sections, but the part from the river up to the railroad was almost unbearable.