The National Forests in WI are the Nicolet and the Chequamegon.
More info here:
http://www.fs.fed.us/r9/cnnf/
If the road isn't on the new Maps, you don't belong there.
Countys that actively log have forest roads (and Forests). Nothing in Southern Wisconsin, and nothing more exciting than a sandy road in most cases. The State leases property to Logging Companies, so you need to watch where you tread.
Here is a list of known Off-Road Parks or areas in WI open to the Public. Fees are charged for all areas except the Pipeline Trail (it's coming). They are easily searched for on the Web for more detail.
* Ma and Pa Rockers-parocker.com/ (Superior)
* McCaslin Mountain-mccaslinmountain.com Not actually an Off-Road Park
per se, but they have access to land (Lakewood)
* Apple Valley Farms-RPM4X4.com (New Auburn)
* Pipeline Trail-In the Nicolet National Forest (Occonto County)
* NorthwoodsOverland-northwoodsoverland.com (NW of Wausau)
* Trollhaugen Ski Area-See Memorial4X4.org -Open on Memorial Day weekend. (Dresser)
Again, I encourage you to join a Club. They will teach you the safe way to enjoy 4WD activities and our environment. Many Clubs also have access to areas, or know of areas that are legal to ride on outside of Off-Road Parks.
Edit: Sean, The State doesn't want 4WD vehicles on their property unless it's on a paved road.