With the back jacked up I can turn the rear driver's tire and the rear passenger turns the same way at the same time.Do I have posi or Ls or locked diff.

 

It'd be Trac-Lok(limited slip)

 

look at the door jamb sticker in the drivers door jamb. under axle there will be a code.
19 – 3.55 Non-LS
18 – 3.08 Non-LS
H9 – 3.55 LS
B6 – 3.73 LS

This will tell you the ratio and if a limited slip or not.

 

I thought LS was a newer version of a Posi?


M V

 

posi is just a kind of LS,(mostly found in GM vehicles)

 

Indeed, Posi-Trac is the GM patented name for their limited slip diff. Ford's brand-name for their limited-slips is the Trac-Lock differential. Trac-Loc and Posi-trac are TYPES of limited slip differentials. There is also the Torsen (gear-type), Detroit Locker, Auburn, and Tru-Trac, or Lock Rite just to name a few. (The latter are lockers, but in my book, they're the only ones that truly limit the amount of slip.)

-Joe

 

AHH HA, you learn something new every day.

Thanks guys
M V

 

If both tires spin the same way while turning one, you have an open dif. If one spins the opposite of the other you have posi

 

An open rear on a pair of jack stands will 'counter rotate, that is, if you turn the wheel one way, the other wheel will turn opposite.

If they turn the same way you probably have a limited slip.

I've done that test 100's of times and the results are consistant.
Chris

 

How hard is it to change out the gearing. I know the 8.8 in the older Mustangs needed to be shimmed and the 9" in my 66 coupe is easier. BTW what size rear is in the trucks. 05 screw 5.4. I have the 3.55 now. Any suggestions to go with either a 3.73 or 4.11.

 

I suspect you have the big rear. 9.75. I have the 8.8 in my 2004.

Everyone seems to agree that the jump to 3.73 from 3.55 is not enough to be worth doing.
They are probably right. I will eventually switch my 3.55 to 4.10 when I get nearer out of warranty.
I don't know how hard it is to do on the 9.75, but suspect with the rigid frame, and various problems with the shaft and LS that you would be better off having it done somewhere that you get a warranty.
Just a friendly suggestion.