This is very confusing. Plus you just got hosed 9.5% tax in WA if you're on the wet side without even taking advantage of the trade.
Have you tried to sell it locally yet? It takes like 10 min to throw it on craigslist with a couple pictures. Should have had it listed before you flew to CO the first time if you weren't driving it there.
Tell us what kind of truck and miles. If its a diesel in good shape, grampa truck it will brig over book if its 4 wd for sure.
Having it shipped is an option but now you get only 8k net for the truck.
Abou the same if you drive it down and fly back.
Seems your only good option is to sell it. Don't be afraid of listing it on CL. You get some flakes emailing you but it is the most prominent way to sell anything private party these days.
What is it? Heck I might buy it.