“Snake juice” is whisky:
snake juice, n. slang (chiefly Australian) whisky; also loosely, any alcoholic drink.
Oxford English Dictionary.
Although the OED says “chiefly Australian”, my searches on Google Books and the Internet Archive found the earliest uses in the USA. In particular, the following two citations antedate the OED’s earliest (1900) citation:
But its redeeming feature is Esquire May and his little shop around the corner. The ’Squire is an expounder of law, moral virtue and distilled snake juice, and altogether is a jolly fellow.
The Field, 27 November 1875, p. 266. Chicago.
“Thet’s the stuff, dog-gone me ef ’t ain’t,—the reg’lar snake-medicine, boys, an’ it was rale thoughtful uv yer to brung it. Dog-my-cats ef I ain’t afeard I shall be snake-bit to-day ef I don’t hist in some of thet ther snake-juice plum-quick. Here ’s to ye, boys; may yer never be snake-bit!”
Frederick A. Ober (1887). The Knockabout Club in the Everglades, p. 185. Boston: Estes and Lauriat.
Eric Partridge suggested that “snake juice” might have been derived from earlier phrases linking alcoholic delirium with snakes:
snake-juice. Whiskey: Australian: from ca. 1890. C. J. Dennis. Ex see snakes.
see snakes. To have delirium tremens: U.S. anglicised as coll. ca. 1900. Earlier form: have or have got snakes in one’s boots remained U.S.
Eric Partridge (1923). A Dictionary of Slang and Unconventional English, 5th edition, p. 791. London: Routledge & Kegan Paul.
A couple of early appearances of these phrases:
Dick Weldon died of delirium tremens (or snakes in his boots).
The Latter-day Saints’ Millennial Star, 1 October 1872, p. 631. London.
“He may also have delirium tremens, which means delirium with trembling of the limbs. Another name for this disease is mania à potu, or drinking mania, which you just now called ‘jim-jams.’ There are other popular names for it, such as ‘D.T.,’ ‘del. trem.,’ ‘seeing snakes,’ and ‘got ’em.’”
Thomas Wallace Knox (1890). Teetotaler Dick: His Adventures, Temptations and Triumphs, p. 119. New York: Ward & Drummond.
As for Kerouac’s use of “snakejuice” in On the Road, the word may have been adopted as an African-American dialect term, like “jazz” and “blues” (though I was unable to find any evidence of this), or perhaps the word was associated with remote or rural dialects, that the characters might have expected to hear in the swamps of east Texas. But most likely is that the swamps themselves suggested the snakes, which suggested the juice:
We were surrounded by a great forest of viny trees in which we could almost hear the slither of a million copperheads.
Jack Kerouac (1957). On the Road, p. 131. New York: New American Library.