Check out oohva.org for oregon areas.
Here are some links to areas in washington...we've been looking for some good places and this is what we've found so far. There is also a Hummer happening this weekend hosted by the seattle local hummer dealers...if you want info, feel free to pm me or drtyfn... Welcome!
http://www.fs.fed.us/r6/wenatchee/cl...orv/index.html
http://www.fs.fed.us/r6/wenatchee/news/newsmain.htm
http://www.4wheelingplus.com/manastash/
http://www.pnw4wda.org/
http://www.n7ums.adahost.com/evans_creek1.htm