Paul Gowder, in response to a guy who calls himself the 'Postmodern Conservative' and says all sorts of weird things: "Combining postmodern and conservative is like combining enema and lye." This is certainly the standard understanding of the matter, which I think has something to do with why various people in academia are interested in finding clever ways to fit the views together in a way that doesn't cause serious intestinal pain. But I think these two things fit together a lot better than is commonly appreciated.
We liberals are the heirs of a long Enlightenment tradition of creating moral progress through reason. Feminism and the civil rights movement are good examples of how this looks in action. These movements are best understood as presupposing the idea that there are culture-independent facts about what's right and wrong, and that human beings can make progress in discovering them. After all, if all the facts about right and wrong are constituted by what societies approve and disapprove of, a feminist trying to reform the values of a deeply sexist society is automatically wrong.
A lot of postmodernism is about rejecting the ideals of the Enlightenment as hopeless and misguided, just as a lot of conservatism is about opposing these ideals. Certainly, old-fashioned conservatives take their moral views as capturing the truth about objective, culture-independent facts, while postmodernists don't. But if you go over to postmodernist views about moral truth, you deny Enlightenment liberals the external standards they need to justify their moral reforms. If you're in a culture with sexism, racism, and authoritarian politics, defending those things becomes a lot easier.
This isn't to say that there aren't some big differences between them, especially as regards religion. But they've got enough in common that I have to think that the big overlap between 'liberals' and people sympathetic to postmodern views is largely an artifact of demography. Liberals live closer to the marketplace of ideas, and that's where people are trying to sell you postmodernism.