Then your case has been approved. Congratulations.
However, 2 months to receive the decision seems to be too much. I recommend you to do the following steps:
1. Take a photo ID and something proves your address and go to the nearest USPS office. Ask them to look for a registered mail for you within the past 2 months. May be the mail came and you weren't at home so the post man returned it.
2. Check online the wake in days for Chicago office or call them to ask about these walk in days. Then go to them ASAP.
While you are in Chicago office, ask them for the tracking number for the decision mail.
I wish you luck and congratulations again