The bodies of work from both modernism and postmodernism contain works that deal with sudden shifts or breaks in "reality". Characters are faced with events and forces which are either inexplicable or nearly inexplicable, "exciting dread", as Freud puts it, and which are therefore uncanny. 

Kurtz is a character closely associated with the uncanny and there are numerous connecting threads from Conrad's Kurtz to the works of modernist and postmodernist writers. Faulkner and T.S. Eliot come to mind as modernist examples. 

This being said, there seems to be an argument to be made that postmodernism may relate to the uncanny, but may not have a special relationship with the uncanny. 

If you have some particular postmodernist writers or works in mind, we may have an easier time making an argument for a special connection.