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With conventional two joint driveshafts, it is very important to keep the output of the transfer case and the pinion parallel within one degree and in relationship to either of these two shafts. The driveshaft itself should be running at an angle no greater than fifteen degrees. This fifteen-degree limit is a pushed limit, well beyond what most factory engineers would allow. Don't be thinking, "Hmmm, if they say fifteen, I can go to twenty." If you cannot fall within this parameter or are close and want to optimize the life and smoothness of the driveshaft, you'll want to pitch up the differential so that the pinion points directly at the output of the transfer case and use a double cardan (C.V.) type of driveshaft. The net effect of this will be minimal joint angles at the differential end (ideally less than three degrees). This will also lower the joint angle at the transfer case end of the driveshaft. | |
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Unless you do one or the other of these two things, you will have torsional vibration. This torsional vibration is a result of the nonuniform rates of acceleration and deceleration through the elliptical path of the universal joint (that's easy for US to say). In some cases, you may not be able to feel it, but I can assure you that it's there. You just can't argue with physics. You might think that this vibration is something you can live with, but you are building and releasing torque two times per revolution on the driveshaft and other power train components. This pressure has to go somewhere, so it causes the drive train components to flex and distort two times per revolution. Eventually, this will cause fatigue failure. Many times people will install very beefy driveshafts and do the most extreme trails with no problems. Then, while pulling out of their local supermarket, they'll suffer a broken driveshaft which is only under a modest load at the time. This is a classic example of fatigue failure. | 
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