I would definitely make sure what axle you have. You did not mention if you have a F250 (Sterling 10.5), F250HD package (Dana M275), F350 SRW (Dana M275), or F350 DRW (Dana 300). There are 3 different axles now depending on which one you have and unless you have the regular F250 without the HD package, non of the other ones have the old Sterling 10.5 rear end. Heres a quick description of the new axles.
"The release of the 2017 SuperDuty brought about some revisions to the axles in order to support the additional power and towing capacity compared to the previous 2011-2016 model year generation. Up front, its basically unchanged with the use of a 35 spline Super Dana 60. Out back, however, there are several variations depending on the capacity of the truck. For those with an F-250, you'll be sporting a similar Sterling 10.5 that has been used for years, still featuring a 10.5" ring gear. Moving from there, the F-350 Single Rear Wheel (SRW) models get a Dana M275 with a 10.8" ring gear and the F-350 Dual Rear Wheel (DRW) versions are bumped to a Dana M300 with a massive 11.8" ring gear. The 10.5" has 35 spline axles, where the M275 has 36 splines, and the M300 boasts 37 splines. "