Jonathan Mayhew questions whether there is such a perceptible difference between the "modern" and the "postmodern," both in fiction and poetry, as we are sometimes led to believe. As Jonathan observes, "the term [postmodernism] took on a different meaning after Lyotard and Jameson. Basically, the word was hijacked as a term for 'poststructuralism' or for 'late capitalism,' respectively."
It is certainly true that "postmodern" was transformed from a term descriptive of an identifiable set of writers and specific literary practices to one that increasingly took on heavy historical and philosophical baggage. Critics such as Leslie Fiedler and Ihab Hassan initially used the word to track developments in 20th century literary history, and it became an umbrella term to gather together "experimental" writers such as Donald Barthelme, John Barth, Robert Coover, etc. Theorists such as Lyotard and Jameson did indeed appropriate "postmodernism" to their own extra-literary agendas, although theirs did not become the widespread use of the term until well into the 1980s. Prior to that time, "postmodern" was mostly an adjective attached to "fiction," a formulation that could encompass both of the nonrealist strategies invoked by Robert Scholes as "fabulation" and "metafiction."
Jonathan suggests that poets from this period were really "continuers of a tradition" extending back to Williams and Pound, that these "new" poets' work still essentially belonged to the "modernist period." I think the same is true of what was postmodern fiction. "Post-" modern meant not just after modernism but more specifically a return to the spirit of modernism understood as the attempt to expand the possibilities of form and style in fiction, an endeavor that to some extent had been interrupted by a resurgence of realism and naturalism from the 1930s to the 1960s. Insofar as writers such as Barth accepted "postmodern" as a meaningful label for their work, they almost always themselves situated the work as a continuation of modernist experimentation and epistemological skepticism. This may have led in some cases to formal experiment that called into question the stability of all narrative conventions, that stylistically exceeded the limits of "fine writing" and comedically deflated fiction's pretensions to transparently representing "reality," but however much these practices might have seemed to challenge the less audacious experiments of the modernists, they were ultimately as much tributes to the inspiration provided by the modernists as attempts to displace modernist fiction.
There's no doubt that "postmodernism" is now overloaded with the connotations of cultural change brought to it by the likes of Lyotard and Jameson, so much so that its utility in measuring the continuity of 20th/21st century fiction--or its disruptions--probably has been lost. There still persists a tradition of anti-traditional writing, but to identify new works that exemplify this traditon as "postmodern" no longer offers much illumination. It only reinforces the stereotyping that passes for critical thinking among critics unsympathetic to experimental writing and links that writing to bloated theoretical speculations with which it has almost nothing in common.