I actually think this is pretty plausible. While these days we think of there being huge differences between the agrarian economy of the South and the New England merchants, the Eastern Seaboard colonies were actually pretty united, culturally and politically, going back to before the Seven Years' War. There was even a plan by colonists to set up a single government for all the colonies in 1754; if you're looking for assets for a TL, there's a political cartoon from some guy at the time saying the colonies must "JOIN, or DIE," which even though it was referring to defending against the natives could be reworked to be a bit of pan-colonial revolutionary propaganda.
I think you just need Britain to really mismanage the Colonial Crisis. There was plenty of unrest in Virginia, too, and the British were pretty desperate to raise funds for a while there; a couple more really bad taxation acts that affected people outside of New England, combined with a general sense of colonial unity, could tip Virginia and/or New York to the Bostonian side, and the rest would follow. It would still be a bit of a New England-wank to watch all the other colonies nobly come to the defense of what would still mainly be a Massachusetts problem, but stranger things have happened, and I'm personally partial to New England-wanks.
It's not until the invention of the cotton gin that the South would really start drifting away from New England. Then slavery would become a real issue; I can't imagine a bunch of Boston abolitionists being fine with the Southern half of their country built on slavery, which would dominate the economy of the whole new nation. Maybe they rebel again!