As I understand it, this is not really a compare or contrast sort of a thing. Epistemology is, of course, the study of how (or if) knowledge is possible. Postmodernism is a set of ideas about the world. As such, they are not really things you can contrast.
Instead, I would say that postmodernism has a specific take on epistemology. Specifically, I think that postmodernists kind of think that you cannot really ever know anything for sure. Therefore, I would say that you have to say that postmodernists have an epistemological belief -- one that says true knowledge is impossible.