Well several of the manuscripts from the Dead Sea have YHWH in Paleo-Hebrew letters in the midst of Babylonian Hebrew text (although I can't remember which ones exactly from the top of my head), and the curious manuscript 4QSam c (
4Q53 Samuel) uses four large dots • • • • in place of YHWH in its text.
My guess would be the importance attached to YHWH's name, and that the idea of "preserving" it was high on the minds of the Dead Sea Sect.
As for the early (and even later) LXX manuscripts: I would not suspect them of anything other than being Greek translations of a different Hebrew "text-type" compared to that seen in the Masoretic Hebrew.
As I've quoted before in an update post, here's what the scholars who produced the "Dead Sea Scrolls Bible" had to say concerning the LXX/Septuagint (in the summary to the text of Samuel):
"These manuscripts have also helped to realign scholars’ assessments of the value of the ancient Septuagint translation. Traditionally, when the Septuagint differed from the Masoretic Text (which had been considered the Hebrew original), the Septuagint was routinely thought to be a “free” translation (or even a paraphrase, or just plain wrong). The Hebrew manuscripts of Samuel found at Qumran, however, very often agree with the Septuagint when it differs from the Masoretic Text. This demonstrates that the Septuagint was translated from a Hebrew text form similar to that of the Qumran manuscripts. The problem in assessing the Septuagint, as with so many historical documents, had been with scholars’ vision and criteria, not with the data. The Septuagint, of course, just like the Masoretic Text, the Dead Sea Scrolls, and every other ancient manuscript tradition, does have its share of errors. But the important lesson here is that the Septuagint is not a free or false rendering, but rather a generally faithful translation of its Hebrew source."
There's a reason why one of the new parts of my
Septuagint page is
Reconstruction of the original Hebrew underlying the LXX - it's time the world knows that there was a different Hebrew textual tradition that was translated by ancient Israeli's into Greek, demonstrating that the Greek exhibits an older, and more faithful, rendition of Yahuweh's word to His people.
The problem has never been with the Septuagint on the Greek translations and/or manuscripts: it's been with other peoples biases