Iowa... There is a nice place about 20 min south of Cedar Rapids called 600 Acres(Hoosier Creek). This is a nice place for stockers and modified people to play. Hardcore drivers may find it a bit lacking tho.
Indiana... BADLANDS!! This place is great for stockers to hardcore rock crawlers. It is about an hour north of Indy in a town called Attica. BADLANDS
Wisconsin... Up north is a place called "The Pipeline". It borders the Nicolet National Forest and has something for everyone, from rock crawling to mud holes that will swallow a truck sitting on 44s.

600 acres is a pretty fun place to try your 4wd out without the worries of pissing anyone off. Just a heads up if you get stuck and no one is out there don't expect a tow truck to come down there. You'll have to head into the local town (Solon, IA) and probably drag a farmer out of a bar, after buying a few rounds, as was the case for me one day. I do agree Hardcore drivers will probably find the area a bit lacking as space is limited to 600 acres making it possible to travel the entire area in less than half a day.