Hey guys. My rear axle code is 14J. Which I know means it has a 9" rear end with 3:1 ratio and front power steering. What I don't know is how many splines is in the yoke. This is my project and I want to put 3:70 or close to that in the rear end. Do you guys have any good advice on swapping to a limited slip diff around that area of ratio and what brands are good? I'm wanting it to be a clutch LSD because I'm terrible at wiring (e-locker) and I would have no idea what I'm doing with air pressure (air locker). This will be my first real upgrade since I bought the truck and any info on ease of installation or part #s or anything would help.

 

is the factory ID tag still on the differential center section? If not the best way to determine number of splines is to pull an axle.

 

Yes. Here's a picture. I don't know how to read it so any help and maybe a lesson in these tags will help. I'm always trying to learn about everything on my truck.

 

If you're talking about the pinion shaft splines, I'm pretty sure all 9" Fords are the same. If you're talking about the axle splines, you have 31 spline axles.
WDM-CF = Ford 9" non-locking w/ 3.00 ratio, uses the #88 service parts list. From the tag you have a '79 (9ED) 9=year, E=May, D=4th week. In '79 Ford used two different size bolts to retain the brake backing plates to the axle housing, either 3/8" or 1/2" bolts were used. The WDM-CF used either so you would need to visually check to see which ones you have.
According to the parts list the 3/8" equipped housing used axle P/N D8TZ-4234-D and the ones with 1/2" bolts used axle P/N D6TZ-4234-A both axles are 31 spline.
Ford did offer a 3.70 ratio ring and pinion (P/N B7A-4209-M) but I like Yukon gears myself.

 

Holy crap! That a hell of an explanation! So all ford 9" pinion spline counts are all the same? And the axles are 31 splines. Ok. Thats wonderful! Thank you!