This seriously needs to be stickied in a few places....
Anyway, here is a link to a great map of offroad parks in the U.S.
Off Road Lands - Parks and Trails by Location
This is a really good listing by state, and seems to be a pretty comprehensive list... I am very sure it is not 100% complete by any means though, just a very very good source.
That being said, I would also totally suiggest making the road trip across Illinois to get to the badlands. It is the best 4x4 park in the area by far.