did thomas Jefferson really say "If a law is unjust, a man is not only right to disobey it, he is obligated to do so."?

The quote was not part of Jefferson's letters and there is no date of origin or when it first appeared in print.

It appears to be based on MLK's famous letter from Birmingham Jail circa 1963: 

"One has not only a legal but a moral responsibility to obey just laws. Conversely, one has a moral responsibility to disobey unjust laws."

A few years later, a similarly-themed incident happened during the Vietnam War in which Lt Willam Calley was charged with murder when he raided a Vietnam village and killed the inhabitants.

It was known as the My Lai (pronounced "mee-lie") Massacre.

His defense incorporated the "I was just following orders" concept - unsuccessfully. It was subsequently determined that a soldier is obligated to disobey illegal orders.

He did indeed. I was there and can corroborate it. 

No, he did not.  It's not known when this false 'quote' first got started but it, like so many other 'quotes' that were never said by those who are claimed to have said them, has spread like wildfire on the internet.

It might be a rough paraphrase of what Jefferson actually DID say in the Declaration of Independence - 'We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.--That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, -That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government...'

This false quote is closer to what MLK Jr said in his 'Letter from Birmingham Jail' -  “One has not only a legal but a moral responsibility to obey just laws. Conversely, one has a moral responsibility to disobey unjust laws.”

Probably not. See this article:

https://www.monticello.org/site/research-and-colle...