I assume you mean ''The Slab'' which is near Zillah. (Or Buena; both towns are wide spots in the road) It's a rat maze to find, but once you've been there, it easy enough. The last part of the route skirts an orchard, and follows a ditch road. It's named for a large concrete slab that looks like maybe it was a barn floor or something. It's a great place to 'wheel, and is about an hour west of here.
As for TSF... ??? A Google search finds "Tillamook State Forest' which is at least 6 hours from here. There's an ORV park in Morrow county, but even that's 3 hours south.