“Unitarian tendencies” existed - that is, various doctrinal deviations within the Congregational church - but these various tendencies didn’t all emerge, nor were they all held by individuals (as opposed to individuals holding perhaps one or two of them), until well after 1776.
Unitarianism didn’t even emerge as a movement at the earliest until 1805 when an important chair of theology at Harvard was filled by a Congregationalist with some Unitarian tendencies. It didn’t coalesce into a denomination until 1825, although William Ellery Channing’s sermon at the investiture of Jared Sparks in 1819 marked the point where all the Unitarian heresies were self-consciously presented as an alternative to orthodox Congregationalism. So, intellectually, 1819 is a clear starting point for Unitarianism, while institutionalally 1825 would be the date.
No signer of the Declaration was a Unitarian or a Universalist. There may have been a couple of signers who had some affinity theologically for some element or other of Unitarianism or Universalism, but that doesn’t mean that they were Unitarians or Universalists any more than by agreeing with a plank or two of the Communist Manifesto you become a Communist.
Well you can see from my post #70 ( http://www.freerepublic.com/focus/f-news/2546951/posts?page=70#70
that Abigail Adams self-identifies as "Unitarian" in an 1816 letter. Granted, this is long after the declaration of independence and also after her husband's Presidency, but it is prior to the 1819 date that Achilles cited for Unitarians seeing themselves as a seperate movement.
The basic point for me that it appears the Adams' church parish adopted a Unitarian philosophy by the 1750s, and that both John and Abigail were registered members of that parish and financially supported it and agreed with the church's teachings.
As for whether specific Unitarian idealogy developed later, I can't say. While I don't mean to insult anyone on FR, though, it seems to me the only "dististive" belief that Unitarianism has is that they DON'T have any specific beliefs!
It is my understanding that a buddhist, an agnostic, a Christian, and maybe even an atheist could join a Unitarian church and become members in good standing with their beliefs. Unitarians profess that they don't have the answers, only that they reject the idea that traditional Christian teachings are certain truth. Some Unitarians still self-identify as "Christian" (as in the "Jesus is my role model in life" sense), but even many of them would probably say they don't accept the traditional Christian beliefs of the trinity, resurrection, etc.
It appears to me that the Adams', while nominally "believers" in Jesus, rejected the idea that he was divine and the rest of mainstream Christian beliefs. That makes their church "Unitarian" in my eyes as well as theirs. Traditional Christianity -- mainstream Catholics, Protestants, and Orthodoxy, wouldn't consider Unitarianism to be Christian. And it certainly isn't related to Mormonism or anything else that has a distinctive set of ideas about Jesus. The fact Unitarians are silent on religious questions and tell members they can believe whatever they want is the core basis of Unitarianism, in my view. It's why they were no longer accepted as a part of traditional protestant churches.
The Green Party may not be "socialist" on paper, but an overview of their platform makes it clear to anyone that reads it that they subscribe to a socialist idealogy.
So we can debate over the details of exactly how and when the Unitarian church was established as a separate denomination, but it seems clear to me that the Adams' family believed in Unitarianism.