Non-Raptor 2013 F150s could come with either a 9.75", 34-spline rear end or an 8.8" 31-spline year end. The easiest way to tell what rear end you have is that 9.75" 34-spline rear end has a 12-bolt cover with an oblong shape, while the 8.8" has a 10-bolt cover and more uniform shape. Your best bet is to count your cover bolts, though the majority of 4WD F150s from 2011-2014 came with 9.75" rear ends.