I asked this question on another board one time, and was told it's much more difficult to narrow a rear than a front. The reason is the ujoint absorbs any misalignments. Alignment for the rear is much more critical. If your end flanges get warped while welding them on, your bearings won't last long. You have to use a jig to make sure that doesn't happen. Here is an article that details making the jig.
A forum community dedicated to custom off-road vehicle owners and enthusiasts. Come join the discussion about trail reports, builds, performance, modifications, classifieds, troubleshooting, fabrication, drivetrain, and more!