If you check Craigslist and search under "Jeep Mud" you'll find a link to a guy in Xenia that apparently has some acreage that he's turning into an off road area. I don't know how many acres he has, but I don't think it's all that big. He only allows Jeeps one or two weekends a month.
Also, there's a place down by Oxford / Hueston Woods that's the same way. Some guy with a farm that allows Jeeps. It used to be called "White Cross" but I don't know what it's called now. Careful down there though - I went with a group about 18 months ago. When we got there, the (half-drunk) guy told us it was $10 per vehicle and if we got stuck it would be $20 to have him bring the tractor over. Later on he tried to get us to buy burgers and dogs off his rusted grill. When we turned him down he then tried to get us to pay $10 a person (said his wife told him that he didn't charge us right when we got there) and then told us it was $50 to bring the tractor out if we got stuck. Haven't been back since.
RnEmOvr - when are you coming back from Afghanistan?