Try Coal Creek ORV area, just west of Oak Ridge.

Coal Creek Area

Just went to the site and they no longer post a map of the area like they used to. I have it in PDF form if you're interested. They do have 4X4 truck friendly trails along with ATV-only trails.

Also, some of the folks in east TN and Georgia apparently know of a place around Helen, GA, not to far south and east of Chattanooga.