There is a pretty large group of dirt riders who hang out on klrworld.com. Most of these are just into NC which is also where the DuPont forest referred to above is.
Some buddies of mine ride in the state areas where you pay (I think $10/day?). We've 2 in upstate SC. There are good areas in the Jocassee gorge which are open to dual sport bikes some of the year. You'll get information on that from the klrworld site, too.