I have gone through the rear axle identifiers to determine that my "13J" code axle is a Ford 9", 2.75:1, and I'd like to get a limited slip, I just don't know if I need a 28 spline or 31 spline. Any help or advice would be greatly appreciated.

 

Most but not all 1/2 ton 2 wd trucks used 28 spline axles. Really no way to tell without pulling an axle and counting splines. But it only takes about 20 minutes to pull an axle out and replace if you have air tools. I have nothing to back this up that is official but I have worked on alot of Ford trucks over the years and this is strictly my experience speaking.

 

IF you have the tag off of the rear, post the code and I can look up which axles are in your truck, but like gatorfor88 said, it's pretty simple to pull the axle and count the splines.
Five lug nuts, pull the brake drum, four axle retainer nuts, yank the axle.

 

No tag attached, but I do really appreciate the advice, I'll go yank it and try my hardest to count higher than 10. What can I say, I only have but so many fingers.

 

Don't forget your toes .
This is what you'll see when you take the wheel off. The axle retaining nuts are accessed through the hole in the axle flange.
A little tip. If the axle doesn't want to come out, make a poor man's slide hammer. Put the brake drum on the studs backwards. Turn the lug nuts around so the flat side is towards the axle and thread onto the studs by three or four turns. Then use the drum as the slide weight and bang the axle out.
Usually though, the axle comes out without too much trouble.

EDIT: What part of FL are you in?

 

Awesome advice, I really appreciate it. I'm in Pensacola, FL right now, hopefully will be here for another year or so depending on Uncle Sam.

 

You're a little too far for me to help in person, I'm just south of Disney.