If you can look for the 2021 superduty order guide that is posted on the blue oval forums by ice capades , go to page 11 and the rear axel codes are listed. f 350 DRW with 6.7L diesel the 3.55 limited slip order code is X3K. it is an optional rear the 3.55 non limited slip is standard. the VOC would show the axel code that was ordered . Ask your dealer for a copy of the VOC to confirm that the correct axel ratio was ordered. don't wait of the window sticker to confirm as it will be too late to change at that point if not correct.