It is always dangerous to generalise and find literary techniques and styles for all the novels of a particular era as writers and texts always retain a certain degree of individuality. Yet, with this precaution in mind, we can draw a distinction between novels written in a modernist and those written in a postmodernist vein.
The 1920s were the years of Modernism, a time when writers began to experiment with literary form and narration. They explored the psyche of their characters and often employed techniques such as the stream of consciousness and flashbacks that undermined the linear, chronological and logical progression of the narrative (see, for example, The Sound and the Fury by William Faulkner). The narrator becomes unreliable and, contrary to realist novels, often does not make any claim to be objective or omniscient. Nick Carraway in The Great Gatsby, for example, tells the story of Gatsby from his point of view. This loss of narrative certainty is mirrored by the themes of American novels written after the First World War, where the impact of the tragedy of the conflict is apparent. Novels such as The Great Gatsby and The Sun Also Rises reflected on the end of an era brought about by the war.
Parallel to this experimental strain, a group of American novelists such as Richard Wright, Nelson Algren, James T. Farrell and the writers of the Harlem Renaissance wrote texts trying to combine literary experimentation with committment to social change.
The novelists active after the Second World War took the modernist experiment to its extreme and reacted against some of its features. Postmodernist writers focus on the sense of play and self-reflexively called attention to the literariness and fictionality of their works. For all the fragmentation of their narratives, modernist writers believed in the possibility of exploring rationally and scientifically human actions. Postmodernism rejected this faith in human thought (you may see here the impact of the nuclear and ethnic holocausts brought about by the Second World War). Works by Pyncheon, Vonnegut, Barth are constituted by fragments which never quite fit together and that make no claim to represent external reality. On the contrary, they challenge the existence of a unified, single reality in favor of multiple perceptions.