Help with Existentialism and Postmodernisim

Mcadge, IRRATIONAL MAN, A Study in Existential Philosophy by Wm Barrett, now on my 3d reading and I still don't understand what it's all about, much less the difference

I looked up "postmodernism,"

http://onelook.com/?w=postmodernism&ls=a&loc=home_ac_postmodernism

but then had to look up "modernism"

http://onelook.com/?w=modernism&ls=a

Hope to get back to you

So here I am. One immediate diff: Post is part of Ex
Isn't it

Yes yes

1 - Existentialists are umbilical while post modernists forgot about the umbilical cord at all.

2 - The first guys celebrate the failure of man while the second ones blame it on nature.

3 - Existentialists want to "get out" n be freed from the cuffs of being inside a body while post modernists don't know where they are if anywhere.

Bonus reply:

4 - Both groups are insane, but existentialists think insane its cool while post modernists think insanity is PI's fault...

Having read around this subject for the last few years, I would sum up the main differences as revolving around the status of 'the observer'.
For Sartre the observer looks outwards and contemplates 'an external world' in all its often trivial detail of arbitrary value.
For Heidegger, the observer (Dasein/sense of 'self') is often absent, and 'reality' is about a seamless 'automatic coping'. It is only when 'self' consciously agrees to choose its 'path' that 'authentic living' results.
For Derrida, the status of the observer is nebulous and context specific. The 'self' which has just written this reply will no longer be that same self tomorrow, and nor will the 'meaning' of this text be the same to the author, or tomorrow's reader.

Obviously, given the iconoclasm inherent in these positions which all embody the issue of semantic shifting, any attempt at a definitive 'answer' to you would itself be problematic !

Fil, as an apodictical existential pantheist I'd take issue with that observation

....except that I'm not entirely sure quite yet what it is; and indeed I've asked myself repeatedly if I might be nuts