The difference is in the thickness of the ring gear. You will need to specify the ratio to the spool vendor, so that you get the right one. In the old days, we used a spacer with the stock PowerLock carrier to space the truck (4.88) gears closer to the pinion center line. A weak solution as eventually the longer ring gear bolts would loosen and cause mischief. But then we also used OEM axles and they began to show twisting signs