I have a 06 F150 FX4 I just wanted to know if these truck have positive differentials and what size are they

 

I think there all limited slip

 

Here is a link to a thread about rear axle size.

https://www.f150forum.com/f4/what-si...ear-end-33620/

 

You most likely have a 373 LS check your axle code on the sticker in the drivers door jamb...373LS is code B6

 

Why is it that on vids u see a f150 burning out and both tires spin I tried doing it and I got only 1 wheel to spin

 

If you only got 1 then the LSD if you have one may be going bad. If no LSD that's why it only does one.

 

Your LSD clutch packs may be worn out. The stock LSD is not exactly the greatest traction device but better than an open diff. My old truck used to get stuck or i couldnt climb much on the slightest 4x muddy hill climbs. I was disappointed then my mechanic repacked my LSD and it was much better off road. You'll notice that when people regear they usually also opt to put in a better LSD such as a TruTrac or ARB locker.