Is it a crime to hide your car that is in repo in IL?

Yes, it is called hindering a creditor, or hindering repossession. It is not likely that you will be arrested for such, but it is far more likely that the creditor will obtain an order of replevin. If such occurs, the repossession agent is likely to return with this order accompanied by a law enforcement officer who will order you to surrender the vehicle. If you refuse you will be arrested, and you will have to surrender the car to secure your release.