Ignore Crepello's response. I think he is confusing St Louis with ORD!
Depends on other arrivals but worst I've waited at ORD immigration was 30 minutes. Quite often it's under 10. The monorail is very frequent (every 5 mins?)and takes a couple of minutes between the two terminals.
Man in the back's advice sounds good and safe. You'll probably have time for a beer.
Of course going backthe other way, you arrive and depart from the same terminal (if you are travelling AA).