It sure sounds like the spring seats (saddle) on the axle have been modified. I can think of no reason why they'd be different. Please be sure that there's a POSITIVE way to locate and retain the spring leaf pack on the saddle, not just the u-bolts. The center bolt should fit into a hole on the saddle. It's a safety thing.
The caster shims on my front axle have a pin on the bottom and a blind hole on the top for the center pin, so the spring position fore and aft is the same whether there's a shim in place or not.
