There is Wharton State Forest, which is on the other side of Atco, just off of 206. Colliers Mills is just on the other side of Fort Dix at the intersection of E. Colliers Mill Road and Hawkin Road. Stay on the trails in both places. Wharton has more challenging terrain, but there are paths around all of the stuff that would give you trouble. Colliers Mills doesn't have anything really challenging, but it's still a good time and very scenic.
Here's a map from Marlton to the Atsion Ranger Station @ Wharton:
http://tinyurl.com/342pcp5
Not sure what might be closer to you in Marlton, never been off roading anywhere over there.
- Justin