I think there is usually a change in noise/vibration/feel from loaded to unloaded. Might be it's too subtle to differentiate. Otherwise I am inclined to agree with subierubieroo on the vast majority of time it is probably outside the driveline. The fact your driveline was serviced immediately beforehand points at the driveline and in your case possible indexing.But the drivesahft is always turning as long as the car is moving, i don't see how being in neutral would make it more "balanced".