First of all, epistemic relativism is much less dangerous to science than it appears. Based on my (cursory) interactions with them, I would say that most historians of science are epistemic relativists…but this doesn't keep them from believing that science exists and is (broadly) accurate!
So how do they square this circle? The usual way one accepts science without epistemic certitude is to think quite carefully about the context-dependency of facts.1 Some information is quite particular — only accessible through unusual circumstances or elaborate sensory apparatuses. The more particular a phenomenon, the more illegible it is: the harder it is to convey to someone else, or to integrate into a larger theory of the world.
From this perspective, science is the process of making local, specialized, contingent information (for which epistemic relativism is essential) into information that is transferable, accesible, and universal (for which it is not). For example, a Thai fisherman might know that the tides on a Thai beach are higher at some times, and lower at others. This is really only useful to that person and their immediate community; anyone else interested in those tides really can only get qualitative information via word of mouth (which is, of course, untrustworthy). If the fisherman were to measure those tides with a convenient stick, then the word-of-mouth effect is removed: they can send the stick to another natural philosopher, who then compares the stick length to his own local tides. And if the measured heights of those tides were written down and tabulated, well…that fisherman's metis (in the sense of Scott) might end up incorporated into a universal theory of gravitation!
Of course, there are other perspectives on epistemic relativism and its relation to science, some of which are much more critical of scientific "knowledge". I personally don't find these compelling; YMMV. But my point is that the relationship between relativism and science is much less clear-cut than you assume.
Second of all, postmodernism isn't exactly epistemic relativism…although you are probably correct to conflate the two. Keith DeRose described postmodernism as a sort of mood; I would say that it is an…ahem…unusual perspective on what makes for good philosophy. To summarize the division (following Stove):
- Postmodernists are interested in writing statements that are both true and important, and don't care too much about getting exactly right the reasons for why those ideas are true.
- Modernists are interesting in writing statements that are true and indisputable, but don't worry too much about making them important right now.
These are clearly complementary sides of the same discipline,2 but they lend themselves to different research paradigms. Since postmodernists aren't too interested in explicating their ideas, they tend to prefer epistemic theories that emphasize real-world experience (which can verify their conclusions) over theory-building and argumentation. Conversely, modernists prefer epistemic theories that minimize real-world experience, so that their conclusions are still forced upon people living in faraway places and faraway times. This makes postmodernists more inclined towards epistemic relativism.
Unfortunately, I don't know a good example of a postmodernist philosopher who believes in epistemic certitude.3 Perhaps another answer can help with that. But philosophy as a whole has been drifting in the direction of epistemic relativism for a long time, so I think the best you can hope for is something like a postmodernist who enjoys Lakatos.
1 I believe this is what the SEP calls "Epistemic Contextualism".
2 …ignoring the fact that many philosophers don't agree on what truth is!
3 I'm also not a philosopher.
