Ford Explorer Sport Trac (2003)
 

I recently bought an 03 Sport Trac, and it's a RWD equipped with an open diff. I want to swap it for either an ARB Locker or a Manual Ox Locker. My question is I don't know/understand what the spline is. I know for a fact, that my differential is a ford 8.8'' with a 4.10 gear ratio. I think that it is a 31 spline but idk how to check without actually removing the diff cover and wasting the oil inside. Also, is there any recommendations for alternatives for the locked differential? it is my daily driver, so I don't want to buy an automatic locker or a spool. If anything, I would think about getting a Torsen. I just want to make it as offroad as possible without having to swap it for a 4X4.

I've got a Torsen diff in my Level II Ranger and LOVE it!
It's the one thing I wish I had on my 91 Toyota SR5 Ext Cab...

Other than that there's the E-Locker but the Torsen really works well