My wife and I are both born and bread Illinoisians. After reading this thread, we re-watched parts of the episode, and he sounds totally normal to us. Jack is using an authentic central Illinois accent. Believe it or not, there really isn't ONE American accent. We can count at least 4 Illinois accents: Chicago, northern IL, southern IL, and central IL. All of them are different enough that you can pretty quickly pick up where someone grew up just by listening to them for a few minutes.
And that's just one of 50 states.... I believe there is something like 6 major english accents and a few hundred minor accents in the US.
What you are hearing is really just the difference between the Hollywood standard accent (which is most American’s have to learn to speak) and someone who really has a regional accent.
That's not to say I like Jack's character... He's way to much of an loose cannon for my taste, He just happens to speak like a real mid-westerner....
