I would check out Four Lakes Four Wheelers, they are based out of Madison and wheel at different spots in southern Wisconsin. Let me know if you want more contact info.
Don't know how far Stone Lake is from you but we race an enduro there every year and it's great trail. Our club has done trail rides there as well other times of the year. Look up Summit Lake game ranch. They have about 80 miles of tight single track, a moto track, cabins to rent, places to camp and a bar/restaurant.
edit: I was thinking SW Wisconsin for some reason.
You can find small places on county land used for atv riding, that what I do northerpart of the state, I thunk it's legal, trail patrol has seen me before and they didn't seem to mind, I don't think my jeep was too much bigger than te bog can am 2 ups they drive either.