If the gods are smiling you should have no trouble with 2 hours. Having flown internationally to and from ORD numerous times over the years I can honestly say I've never had a problem at all. I've never seen a problem with ORD's customs staffing levels either. From the point I exit the plane until I walk out the door of the international terminal I don't think its ever taken me more than 45 mins. Of course, all this assumes that your documents are in order and you're not randomly selected for search and questioning.