Here's my thinking on how to determine the shim angle needed. Figure a 2 inch budget boost as is the case with most of us on this thread. You used some combo of leaves to achieve your boost (in my case a Qtec HD pack wed to my stock main). Now the lift you gained is from the increased arch which is effective between the rear shackle bolt and the axle (the part of your pack forward of the axle is flat).
The axle, the rear shackle bolt, and the point of intersection formed by a horizontal line drawn back from the axle and a vertical line drawn down from the rear shackle bolt form a triangle. Changing this triangle doing your lift pointed your pinion up causing vibes/rumbling sound, or at least that's my assumption. In the original stock configuration, the arch in your spring placed the rear shackle bolt about 0.5 inch above the horizontal line projected from the axle (my best approximation, feel free to correct me anyone).
Now with a 2 inch lift the shackle bolt is 2 inches above this horizontal reference line. Two inches minus 0.5 inches is 1.5 inches. The horizontal distance from the axle to the vertical line projected down from the rear shackle bolt is 29 inches.
So we have two legs of a triangle at 1.5 and 29 inches. The angle at the point of the triangle at the intersection of the horizontal and vertical lines is 90 degrees, so let's use some basic trigonometry. The angle at the axle whose cotangent determined as 1.5/29 is 3 degrees. You need a 3 degree shim, or close to it, to correct your pinion angle.:looser:
