Between Marxism and postmodernism there exists a common ground: both are opposed to reification of facts, hypostatization of ideas, an ontic fallacy (the idea that knowledge is compulsively determined by being), positivism, etc. Postmodernism, however, fails to sustain ontological realism & judgmental rationalism (for more on these terms, see my posts on Roy Bhaskar, or read Bhaskar himself, for that matter) in its philosophical premises. That's where they differ. However, it's not that one needs to be epistemologically correct, so to speak, to engage in effective politics at all. I agree with Brecht:
***** from _Life of Galileo_, Scene 8
The Little Monk: But don't you think that the truth will get through without us, so long as it's true?
Galileo: No, no, no. The only truth that gets through will be what we force through: the victory of reason will be the victory of people who are prepared to reason, nothing else. *****
Yoshie