[Jeff's bowtie's]

A front end alignment would not correct the axle being 1 1/2" forward or backward on one side or the other if I'm not mistaken it would only change caster & camber as well as toe in or out the axle is mounted to the springs and both should measure the same from front mount spring bolt to center pin bolt if both measure the same only thing causing A 1 1/2" difference is frame out of rack and that much out would show up on body mounts core support mounts.
Or when they put the new springs on they didn't align the center pin up in the axle on one side. Changing rear shackles would not set the axle forward or backward as the front spring hanger is stationary on the frame and both springs are the same as where the center pin is located on both springs