At ride height the panhard bar should be parallel to the ground with the rear axle centered under the truck from side to side. As the suspension jounces and rebounds, the end of the rod attached to the truck will swing through an arc, effectively changing the length of the panhard bar and moving the rear axle to the left or right of the truck centerline. If tire clearance is tight, use the longest bar possible to minimize this side to side movement. This would require fabrication of new mounting points. In your case I would try removing one end of the bar, and then center your axle under the truck from side to side. Hold the bar back up into position, this will tell you whether you need to lengthen or shorten your bar to recenter your axle. Then shorten or lengthen accordingly.
John
