Yes, as Exiled noted, I did specifically say "two most influential and original" 20th Century American Authors. Fitzgerald, Kerouac, Salinger, and Irving (I assume you mean John) I would definitely put on a shortlist of the Great 20th Century American Authors. Ken Kesey is another to be mentioned. Saul Bellow would be on there if he wasn't Canadian. Philip Roth is definitely on there. Norman Mailer. So there's been a lot of great authors. But in terms of the uniqueness of their prose and the influence that their unique styles had on writers that came after them, I would have to say Faulkner and Vonnegut. Faulkner is an absolute no-brainer, imo. Vonnegut takes the number 2 slot because, as Slothrop noted and I would agree with, he bridged the gap between all the post-modernesque writers that came after him and the modernist writers of the past through his combination of so many different genres and styles of writing while still keeping his prose as sparse and succinct as it was.
Edit: I also find Thomas Wolfe to maybe be the most overrated author out there, so he's definitely not on my shortlist.
IMO