Under vehicle by end of torsion bar (side closer to rear of vehicle) there should be a 6-point bolt which when tightened or loosened will apply more or less tension to torsion bar.
Good info about raising front end by adjusting torsion bars. www.duramaxforum.com/forum/04-5-05-lly-performance-parts-discussion/4491-torsion-bar-adjustment.html