Post Modernism is a cultural logic. Post-structuralism is a theory of language. Neither threaten science.
Oh, no. Post-modernism views rationality as a Western cultural construct, a form of domination and control. Post-structuralism maintains that there is no text, only interpretation, and that "authorship" is a myth. As the trendy post-modernists are wont to say, "There is no text, only interpetation" and "There are no facts, only interpretations."
If radical anti-rationalist philosphy doesn't "threaten science," I'm not quite sure what possibly could.