Well, when I criticise something, the biggest fear is always somehow misunderstand the thing I attack, since misunderstanding of the things that are attacked is something I often observe when arguments are made. I'd appreciate if you help me out a bit here!
Again, I'm not a major and has only used this for application. I have an issue with clarity in my first language already so I apologize if this is highly inaccurate or unclear. You must be aware of the difficulties expressing postmodernist thought.
To put it way too simply: Saying that reality is only present "in the text" is not about
what is written, it is about
how it is written. It is not about knowing that the world is 6000 years old, it is about understanding how it is known. In the text. There is, to the postmodernists, no need for objective knowledge; critiquing the lack of objective statements or universal truths is missing the point. Postmodernist epistemology is questioning and analyzing, not defining and describing.
In the natural sciences, texts are ideally approached with a regard of clarity, understandibility, and reliance on empiry (original or accumulated) for the sake of archieving objective truths. But texts aren't always understandable. Phenomena do not represent themselves to us in the fullest.
The natural sciences know this too. Devices for acquiring information are limited in scope and only provide the answer to one question we ask. I'm unaware of the accurate English terminology, but I'm sure it was Bohr who came up with the paradox/problem. The natural sciences just have to attempt working around their limitations, as the approximation of objective knowledge is their function.
Similarly, if you want defined what postmodernism "is" - it really, well, "isn't" anything monolithic. It's a number of philosophers that make sometimes similar, but often very varying conclusions from assuming that the Age of Enlightenment, modernism, isn't a proper tool for understanding anymore.
Postmodern philosophy as such then clashes with the natural sciences that have Enligthenment as their fundamentals. Which is understandable, but honestly, to me, I'd rather use postmodernism in the humanities.
A concrete application of this premise is the very popular
différance - I think Lovett mentioned this half in spite earlier? - which you must know as you seem acquainted with postmodernism.
Différance means two things; first, that the meaning of a word is only created in the reference to another; and following, that such a reference creates hierarchy, binarity and opposition between those words. Inaccurately put, this destroys the possibility of a word having a meaning by itself, having objectivity a very difficult thing to archieve.
What it boils down to?
It's really just good arguments for you to have an open mind, don't be presupposed into a certain epistemology, understand the structures that constitute our framework of knowledge.
To objectivists (not the randian kind) the downing of their hegemony will of course lead them into desperation, believing that humanity will suffer without objectivity; relativism is bad n' all. But for each Bible-cosmology-scare, you'll have better anthropology, better understanding of culture, arts and the different movements, meanings and thoughts that are presupposed to each of those fragments.