I got a posi unit from my brother who thought it would fit his car but it's the wrong year. I need to know how to check how many splines my axle has before i give him any money for the unit. I don't know exactly what i'm suppose to ask he just called me from work and told me something about splines and axles.lol...I'm sure he'll edit this post when he gets home and ask the right questions. He just wanted a few responses so he wouldn't have to wait. my car is an 88 formula i'm not sure what year the posi is out of.

 

Count them.

 

He's right, but not explanatory.

There are a few options.
Option #1: remove the rear diff cover and make the first spline, and then rotate that wheel and count the splines.

Option #2: remove the axle from the car and count them.

Option #3: make sure that rear is stock. If it is, do a search and find out how many splines they have.

 

Are you talking about the car in your profile? You could have a 9-bolt or a 10-bolt, further complicating things. Also, you dont have 3.73's stock.