Since postmodernism, what kinds of authors now frequently share the american literary canon that didn't before

That would be the authors from different culture.

Before the the advent of postmodernism and the emergence of the new authors the canon was mostly made up of literature written by white males. As postmodernism took the central stage writers from different cultures and writers who are women have entered the cannon. Also the writers that are ethnic minorities take the central stage like Tony Morrison and Ishmael Reed.