I previously worked in an engineering firm , and had to refurbish a prop shaft from a large tipper truck. This weighed around 70 kg and had a small 10 gram metal balancing piece tack welded to it. Not knowing any better I ground this off, and got a right bollocking. The boss said he would have to pay for it to be re-balanced. If this is so important on a waggon then surely the same principle applies to driveshafts on cars, you would expect the machining process to naturally balance it out, but it would only need a slight deviation to set up a resonance vibration . This would happen at certain revolutions which may differ for individual shafts.