You really shouldn't worry at this point ... just head to the airport early and they will assign a seat.

This question gets asked every couple days (sometimes daily) on this forum. Remember (or if you didn't know), that a certain number of seats are held back for airport control. They do this to accommodate people with disabilities, families, etc.

Also, your profile doesn't indicate that you have status with AA. The exit rows are only selectable by elite members, so if you don't hold status, you won't see those seats either. I did a check of FLL to ORD on ExpertFlyer and see that most flights still have availability, so I don't think you'll have a problem if you get to the airport on time.

Have a nice trip!