Slightly further north, but still in IL, there is a clinic at Lutheran General (I'm going to get assessed this week.) Dr. Nancy Landre is the main neuropsychologist.
I found her by searching for neuropsychologists in my health insurance's find-a-physician (took me FOREVER to figure this out, turned out the answer was much simpler than I supposed.) You will find that doctors in that field who work with adults with brain injuries, epilepsy, and muscular dystrophy may also be able to diagnose autism, and if not, will probably be able to refer you.