Because of the changing speed of the drive shaft the two u-joints (one at each end of the drive shaft) must be in phase. Each one must speed up and slow down at exactly the same time as the other. This is something to consider if you ever take your drive shaft apart, or it falls apart on the trail because the slip spline is not long enough for your vehicle's needs. Care must be taken when putting the two pieces back together so that the u-joints are in phase. If they are not in phase it will create unnecessary driveline vibration
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http://www.4x4wire.com/toyota/tech/driveline/
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