If no one's told you about it The Cliffs Insane Terrain Off-Road Park is in Northern IL AND it's legal.
Here's a link http://www.thecliffsinsaneterrain.com/
You can get there from Buffalo Grove in around two hours.
Take 53 south to 355 south to 55 south to 80 west. Get off at Rte 47 and head south a bit, not too far, to westbound Rte 6 . Route 6 will take you right to the door.
It's a nice place, but not for ill equipped or showy rigs. Most of the trails are very technical, very tight and very off camber. In fact some of the trails are really just narrow ravines that have had some of the trees removed. If it's been at all wet, it will be very gooey and slippery. The chance of body damage is high if your driving and tires aren't up to the task. Really aggressive mud tires are a must, Interco being the brand of choice, Swampers, Boggers, TSL's, LTB's, Trxus are all good, but not Thornbirds (the staff trucks all run Boggers as do many of the regulars). I have run it with radial mud tires and have done pretty well, and I've seen others running MT's there.
It's the closest place to you, and The Badlands is the next closest.