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From the copy of the service manual I have (HD).
Truss rods are installed so straight portions adjacent to differential housing castings are parallel to axle tubes. This position may be obtained by loosening fitting on low end and tightening fitting on high end. When truss rod bearing surfaces are parellel, tighten both ends an equal amount until rod is seated in notch in housing.
Caution: seat rods just firrmly in housing notch; do not draw tight. Drawing rods tight will deflect or bow housing and axle, and wear parts rapidly.