Andrew, you’re redefining the word “progressive” rather loosely. A progressive fights to implement social reform while a libertarian is merely ambivalent about social issues. There’s a world of difference between the two. Libertarians believe in a limited role of government, which is exactly what conservative Republicans believe. Gary Johnson spent his political career as a Republican, so it should be obvious which political party he would caucus with if he were president. And it ain’t the party with a progressive platform.
Also, if marijuana legalization is such a priority for you, you should realize that progress on this front can only be made at the state level. The federal government will follow the lead of the states, not vice versa. This is the typical course of action as cultural attitudes evolve, just as it was for gay rights, women’s suffrage, slavery, etc.