U joint problem?

[gman]

Need some expert help. When letting foot completely off gas pedal quickly, there is a heck of a bang or slapping sound which sounds like its coming from the driveshaft area. It also does it when accelerating after coasting. There's obviously slop somewhere but I'm not sure whats causing it. U joints? And if so, how do you tell which one? How do you tell a bad u-joint? Any help would be greatly appreciated.