Welcome to Seattle. It's such a great area, you'll sure love it. Check out the Mt. Baker - Snoqualmie National Forest. They have many many fire service roads, you'll not run out anytime soon. Depending on where you are in the Seattle area, try out Stevens Pass area down on HighWay Two- Take the "Old Cascade Highway and turn off on "Iron Mountain Trail". Also, try the Cascade Loop and the Mountain Loop. There are some Sand Dunes in Grant County, one is right off of the Columbia River the other is near Lake Moses, both only a few hours drive.
I don't know of any 4Runner groups, would be awesome to get one going. I know the Tacoma World guys go out fairly often, and they post on here every now and again before they meet up. I think they do a lot of stuff near Evans Creek ORV Park and other trails near Mt. Rainier.
As you may know by now, snow is starting to fly in most areas, Stevens Pass, Snoqualmie Pass, Mt. Baker and Mt. Rainier. So be careful driving up 5000ft on a narrow service road.
I'm always up for exploring some new areas and love going back to old ones. Let me know if you want to hook up.