What year was the F150?
The 9" came with 2 different spline axles, 28 and 31. Your want to look for either the big bearing 28 spline or the 31 spline unless you plan on upgrading shafts later on. The 28 spline shafts have 2-3 holes drilled, 1 each between lug studs. The 31 spline shaft has 1 hole between two of the lug studs. I believe the 28 spline big bearing axles came in the 76 and 77 bronco's. The carrier's will need to be changed out if you go from a 28 spline axle to a 31 spline axle. From my understanding you can make 35 spline shafts fit, but it requires a bit of work. Not to familiar with that part of it.
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