Heavy is the head that wears the crown. But until criminally ambidextrous DJ Z-Trip (whose mixtapes fetch as much as $200 online) abdicates the throne, the short-lived mash-up phenomenon will have one reigning king: Arizona's Zach Sciacca, the crate-digging force behind widely bootlegged copies of Uneasylistening, Vol. I, a shining moment that not only granted AC/DC's "Back in Black" sexy face time with Madonna's "Like a Prayer," but allowed Martin Luther King to stumble into the Eurythmics' "Sweet Dreams (Are Made of This)" with all the snarl of a sleepwalking zombie. As weirdly compelling as that all was, there are only so many times royalty can drink from the well of Darth Vader's "Imperial March" without sounding contrived. What's next? The Mission Impossible theme with George W.'s laughable aircraft-carrier ramblings? Thankfully, Z's Shifting Gears tapped into the cultural zeitgeist with the best of 'em (Danger Mouse, DJ Shadow, Whipper Whip, Blackalicious, Terminator X, etc.) while still managing to keep the satyrs-come-lately feeling sick and dope.