Kyle,
check your rear-end fluid. But with 313K this is probably not a sound to worry about too much as long as all fluids are up to correct levels. Worn bushings and the like could also transmit sounds you may otherwise not be aware of too. I might be wrong about this since I only used to check my Mazdas like this. The center drive-shaft bearing, get under car, grab axle and try to move it. If it's like the Mazda the axle's not supposed to have much flop in it.