Hello, I'm new to this site and have a question. I have a 1994 F150 XLT 4x4 and the door sticker shows my axle code as H9 which I've read is 3.55 limited slip. However, when I peel out or spin the tires, both spin like posi trac. I guess someone who owned it before me put posi trac on it???

 

Maybe they did, or maybe your limited slip is working really well.

 

With limited slip, basically as soon as one wheel starts to turn faster than the other it torques up the clutches and locks them together.
Takes less than 1/4 turn of the wheel if it's working good. A little longer if it's not.

If you're really curious, you can pull your diff cover and look. Picture is what a factory limited slip (trac - lok) looks like.
May as well see what it looks like inside before it's blown up because if you're doing burnouts and peel-outs in a 1/2 ton truck, she's going to blow that rear diff into schrapnel. It's just a fact, it won't take it. Maybe if you put 1000 worth of upgrade stuff in, you could keep doing that.
Just so you know ahead of time.

 

Ok thanks for letting me know before something happens. It's just little peel outs in the mud and things like that but I still better not chance it.

 

You can check to see if you have a limited slip differential. Jack one wheel off the ground and put the transmission in neutral, if you can turn that wheel you have an open differential, if not you have a limited slip.

Method two, jack both rear wheels off the ground, a floor jack under the pumpkin works well. Turn one wheel, if the other wheel turns backwards you have an open differential, if the other wheel turns the same direction you have a limited slip.