If it makes that noise when you're in P or N, it probably isn't the driveshaft (doesn't rotate if the wheels are stopped). Are you sure it's not coming from the engine compartment? When my alternator bearing started going south last week, it made a sound exactly like what you're describing (actually, more like rocks being tossed around in a box, but pretty close).
Or, fuel pump? Should be on right side of car, by right rear wheel.