This situation looks like a case of "tolerance stacking". If one or both axle shafts are a little on the short side, but still within the manufacturing tolerance, and the spacer block happens to be on the small side as well, it's possible the recommended clearance will be at, or beyond the specified limit. Add a little wear anywhere. such as on the ends of the axles, and suddenly the stackup is out of theoretical limits. At the very worst, I doubt if an extra side to side motion of the back end of the truck the thickness of a business card is going to cause any problems.
Jerry
