I think that there are a couple of Postmodern elements in the King short story.  The first would be that the entire purpose or quest to find "the body" of Ray Bower does not yield much in way of transcendental truth.  The boys embark on the quest to discover the body and the entire build up to it is anti- climatic.  Little in way of unifying principles is evident when the body is discovered.  I think that this can represent the Postmodern idea that there is little in way of transcendental and unifying conceptions of truth and identity.  Another Postmodern idea that is evident in the short story is the critique of truth in terms of friendship.  Adolescence, in general, is shown to be a Postmodern element where friendships and attachments are not everlasting and universal.  Rather, they are shown to be contingent and dependent on circumstances.  Gordy does not maintain his connection with Teddy and Vern, who die unnatural deaths.  Chris and Gordy remain friends, but his demise outside of a fast food restaurant through an almost random act of violence helps to reaffirm the lack of totality that is evident in the depictions of friendships.  This is a Postmodern idea and it is brought out through the notion of friendship.  Finally, the emergence of how the individual experience is of the utmost of importance to narrative is a Postmodern idea and is embodied through Gordie's storytelling ability.  Gordie is a Postmodern story teller because he is the author of his own stories, from his own mind and something by which anyone can appropriate, as represented by how Chris is equally successful in the college prep classes he takes.  Storytelling is not shown as something that comes from the divine or from the powers of artistic importance.  Gordie has an ability that is something that he ascribes importance to and is something that Chris is able to share in his own exploits.  In this, a Postmodern critique about how the artist is apart from society is present.