Is it against the UCMJ to become a member of the Oathkeepers?

I don't see how membership would violate the UCMJ. That said, I personally think that Oathkeepers are a little "tin foil hat-ish".

I don't support any group that makes up it's own tab lol. Jus sayin ;) haha

But seriously I do wonder where folks get the time and energy to put into something so far reaching. I'm all for liberty and constitutional rights I just don't need to join a group to do it.

I had a longer reply but decided against that. That you are asking the question shows that you are not sure of this group. Personally, I would steer clear of them as I find some of there declarations to be questionable. I do not question the research that went into what they are telling you but rather that they may not be telling the full story. The military has been used in the past to preserve the Union and that is a precedent that has been set. We are not treading any new grounds today that were not walked my our forefathers. We just need to tread carefully...