No good quad riding in eastern wa. its a dustbowl. Head to oregon. Its only a little farther, and way better riding. Heck, Sand Lake, even tho its pretty small, is only about 4 hours from tacoma. We've daytripped it in the past.
If your dead set on eastern wa, there is a spot just a couple miles south of beverly dunes, along the river but before you get to the dam. You park on the right side of the road, then jump over the road onto some BLM land. There are a ton of old goat trails that the dirt bike guys ride, but they skip this really awesome little piece of sand that goes up into a crack in the cliff wall. Steepest sand dune your gonna find in WA. So take paddles and dirt tires and ride both.