I built the rear in my Duster. From the research I've done the so called "experts" say not to worry about the drive side on used gears. The coast side is more important. I have an Yukon instruction booklet and it says to concentrate on having the pattern centered left to right and not worry about the pattern being up or down. This is supposed to guarantee correct pinion depth. This actually makes sense to me. Go to Randy's Ring n Pinion website. You will find very valuable info.