The shims are there to compensate for different loading of the bed, like maybe a utility bed, or a truck that runs with the bed half full of tools. etc. most of the time. If the axle were designed to one fixed caster setting, it would only be right when running no-load, something only the anal-retentive restorers do with their Cowboy Cadillacs these days. Remember we're working with vehicles that were intended to work for a living 50-60 years ago. Shimming an axle for caster correction was a routine procedure when doing a wheel alignment on a working truck back then. Shims come in all sorts of thicknesses for adjusting the caster.
Jerry
