While your situation is less than ideal, it is a situation facing many "Chicago-land " dwellers. You have 2 options:
1. If you can afford it and have the resources, you can bring the difference between your sales price and what you owe on the mortgage to closing. This option obviously cuts into your cash reserves, but it will leave your credit unblemished.
2. You can try to execute a short sale. I use the word try because you must have a compelling and "documentable " reason for requesting a short sale. Your mortgage company has to review any offer you receive and review the reason behind why you are "selling short". If they determine that your reason is valid, they may accept a short-sale offer. While this is great news for you selling your home, there is a penalty to be paid in a derogatory item on your credit report.
I have extensive experience and a thorough team of short sale attorneys that aid me in successfully completing short sales. If you would like to discuss this matter further, please do not hesitate to call me at 708-932-1352.