One thing to watch for when using such thick shims is the length of the center pin head in the leaf spring. If the head of the center pin does not reach all the way through the shim, you will have no way of locating the axle correctly. As you know, the axle has a hole where the spring pin goes.
Excellent point Joe, and even if you get it right the axle won't have any way of staying put if you hit a pothole, curb, large rock, whatever. Which I think is why the shop manual says it is inadvisable to use shims to correct more than 2 degrees...
Jon
