Yes, Texans say "Y'all," as in "Y'all come back now, y'hear?" I don't think I ever heard this expression in Michigan, where I spent the first twenty-six years of my life. We would say "You guys." I may have heard it a few times in Tucson, where I lived for five years before moving to Texas. I'm pretty sure I've never said it myself, although perhaps one day I will. I'm becoming Texan.
Sometimes you hear "All y'all," which is short, I gather, for "All of you all." Why not "All of you"? Why not "Tejas"? It's the way things are. A few months ago, while waiting for a train to pass, I read the graffiti on the cars. One item jumped out at me: "Fuck y'all." I like to think a Texan wrote it, or maybe someone mad at (or envious of) Texans.