One of the happiest sounds we hear on TV each week is the opening thump-thump-thump that kicks off the credits sequence of FX’s Justified — a slow, insistent lockstep like the cowboy-booted saunter of Timothy Olyphant’s Marshal Raylan Givens, a dude who doesn’t hurry because he never has to. Even better is what follows: a drawled, bluesy lament rapped over a snarl of banjo and dobro, conveying the hero’s tangled rural-urban roots so perfectly you’d think it was written with the show in mind. But it wasn’t: The song, “Long Hard Times to Come,” is the work of Brooklyn’s Gangstagrass, composed of producer/guitarist/beat-master Rench with an ensemble of a dozen other players, ranging from MCs and turntablists to fiddlers. They’re making a Nashville stop en route to SXSW in a Nashville Fringe Festival-sponsored show with The Deep Fried 5, Charles Butler and Associates, artist Openmic and the Music City Soul Dancers. Miss it, and you’ll have to answer to Mr. Limehouse.