Olympia or Portland to Steamboat Springs CO

[nubicy]

Planning my first trip in 9 years.  I'm excited to see all the route maps available, but confess I'm still confused.  I want to follow the Trans Am route through Yellowstone into Rawlings WY, then from there maybe find an alternate route straight into SB instead of going through Walden and backtracking over Rabbit Ears Pass.  Anyway, my main trouble is figuring out the best way to get to the Yellowstone portion.  I'd rather avoid biking way north-east to Missoula, just because of time constraints.  
So, any decent routes from Kennewick to Yellowstone?