seems like you have to get on the interstate for a few hours to get anywhere legally to mud/wheel etc. kinda sucks if you ask me, when i lived in NC there were public/private trails all over the place. exploring the francis marion forest service roads are public, and can be interesting though, and if you go after a good rain its hard not to find mud everywhere, and its legal! just stay on/close to the roads
there are some decent looking ATV trails through there, and last time i was up in there it was obvious full size trucks had been down them, although this is not technically "legal".