Informal contraction of "you will". E.g., "If ya'll send me a dollar, I'll pay ya back Thursday."
Also a common misspelling of "y'all" by people who are sure there must be an apostrophe somewhere but don't know where to put it.
A tip: The abbreviated form of "will" is 'll: he'll, she'll, it'll, you'll, ya'll.
The abbreviated form of ya is y', y'know? (Not to be confused with the enclitic form "yanno", yanno.)
bozon: Notice the subtle difference between y'all and ya'll. The first one, you're right, has no "will" in it, but the second one--the title of this node--is a different word and it does. "Y'all" rhymes with "wall". "ya'll" rhymes with.. uh.. well it's close to rhyming with "full", but 'full' is stressed and "ya'll" is always unstressed so it's not a perfect rhyme.
BTW I'm from Tennessee. Ahyuck.
...Odd, I've never run into such a creature as "yu'll" before.