You get a 31 spline axle that is the exact same length as the factory 28 spline axle, so far so good, now you have to change the posi to a 31 spline setup that has the bigger cross pin and who knows how precise the posi was machined, the ring gear surface could be offset to accomodate lower gear sets, and then your new 3.73 or lower gear needs to be reshimmed to get the correct backlash......all this adds up, the axles will not be in the factory location....and the factory location was probably not perfect to begin with. Some calipers need to be shimmed and others dont, Ive built my share of rear axles and this is just part of modifying cars.