Several thoughts on this
(1) It would help a lot if "relation" were defined more clearly. Do you mean "share similar ideas"? Do you mean that one learned from the other? Do you mean they organize the world similarly?
(2) "Post-modernism" is a pretty nebulous term that refers to a lot of different things, so there's a little bit of something for everyone in it (and a lot of something where they would claim that can't be postmodernism). Let's take it for our purposes to be philosophies and outlooks that come in the wake of modernity and don't share its optimism for the prospects of modernity's project.
(3) Given the two caveats about, I'd say what the two have most in common is that they can both serve as critiques of modernity. Or at least Taoism (and to some extent Buddhism) can be repurposed to that in philosophy.
(4) It's wrong to call Taoism or Buddhism itself "post-modernism", because neither is a philosophical outlook that develops after "modernity." Instead, they are pre-modern in several senses. First, they precede modernity in Asia (and in the West). Second, they do not grow as a response to a thorough-going "scientific" approach to knowledge.
(5) As I suggested above, both agree about being critiques of modernity. But just because different views agree that the "truth cannot be grasped intellectually", this does not mean that they agree that (a) the truth cannot be grasped and (b) that there's no truth to begin with.
(6) Given this, one major point of departure in my view is that Buddhism while not believing that the truth can be grasped intellectually does believe that there are truths, which is not a view shared by the post-modernists. Stated another way, a good post-modernist would look just as askew at claims about a Buddha-nature in all of us or the prospect of enlightenment through the 8 fold path as they do at Kant's claims or traditional Christian claims. This is because they (let's say perhaps more carefully the post-structuralists and Rorty ) don't believe there's something beneath the web of meaning.
To give an example, both post-structuralists and Buddhists think there's a problem with the rational self. Both think it should be eliminated or shown to be illusory. But for the Buddhist, there is something under that even if it is not to be grasped intellectually. We could on some level claim there's a type of truth in the universe for Buddhists. For the post-structuralist, beneath the idea of self, there are structures that are not themselves true nor do things get any better as we keep going down.
(7) There's a different set of differences with Taoism. On my interpretation (which agrees with some of the more popular scholarship today), Taoist thought maintains that humans should live in harmony with nature rather than be caught up in either trying to grasp things intellectually or subdue nature to human will. This is expressed in the very name as the Taoist understanding of Tao. For most post-modernists, this Tao isn't going to be acceptable at the end of the day. It's still a fundamental construct that orders reality -- even if it's not something like the Confucian one or logical positivism.
tl;dr - both critique modernity but they don't agree about everything.