Hi Malc,
I found the post!
Beyond some basic summaries, my only study in Existentialism was to read Martin Heidegger's 'Being and Time' ... although Heidegger was a great influence, he claimed not follow Existentialism. If I recall, Existentialists see reality as absurd which fits into Heidegger as he, for example, described our existence as 'thrown' ... we do not chose our life but somehow 'land' there. I'd have to review my notes but I don't think Heidegger went as far as the Existentialists to conclude life is therefore essentially meaningless or a farce.
Anyway, that has nothing to do with your post!
Despite what is or is not Existentialism, what you are saying fits quite nicely in my thoughts on truth ... to not care is to basically not seek truth and to see truth as only relative (not universal) is basically an excuse to not seek truth.
I think in accordance with what you are saying, I hold that truth is absolute, or at least that it seems to be so empirically (a subject for deeper debate that I could certainly better defend/elaborate). Knowledge, however, is not absolute. That is the rub ... if knowledge is not absolute it is difficult to establish that truth is absolute or universal.
A hint about why I think truth is absolute is the unique nature of consciousness. My experiences are UNIFIED within the conscious me in a way that has no physical analog. It is the unity of consciousness that allows comparisons to be observed and truth is bred from this mysterious capability. Thus, our fundamental and unique nature as conscious beings is to be 'truth seekers' ... to deny truth is to deny the special nature of one's self.
THE common ground to establish a moral society is to want together to find truth. We need only to care more about truth to care more about others, advance ourselves, and live life more fulfillingly.
News 
Market Data 
Discover 
