I am looking to swapping out gears and changing my differential. I have a 9" on my 73 mustang but have one question.

How do I know the axle is 28 or 31 spline? And is there a way to know w/o pulling it apart.

 

Unless it has aftermarket axles in it, you most probably have the 28 spline. It doesnt take much to pull an axle, put it on jackstands, pull a wheel, pull the drum, pull the 4 bolts holding the flange to the backing plate/housing and pull the axle out.