Weird Vibration
 

There is a weird vibration coming from the rear of the car. It only happens on acceleration and deceleration when in gear. It's not strong vibration but definately can be felt. Also it does not happen constantly, often yet not too often. First I thought that it might be the resonance (sp?) from the exhaust when it touches the flooarboard but I doubt it, maybe posi going bad? Any ideas? All input appreciated, thanks.