I am going to change my 3.08's to 3.55. Not a problem, but I have found a posi. that came out of a mustang. I am not shore yet if it is a 28 or 31 spline. The year of the mustang was about a 90 to 93. My question is, what is the spline of the 8.8 on a 99 f150? If the splines are different, can the spider gears be changed from my rear end to the mustang posi? If any one out there can help, I would most grateful.

 

The trucks are 31 spline. I think some (not all) of Mustangs are also 31. If you use the old one, be sure and replace the clutch-packs while you have it out.

 

Sorry, but the Mustang is 28-spline, as are Rangers and T-Birds. Pretty sure it cannot be converted, there are some internal dimensional differences which would be either expensive or even impossible to overcome. You can buy a new Ford Motorsport 31-spline Trac-Lok ("posi" is a GM term, please don't use it on this board!) unit from Summit for $250. You might have close to that in a junkyard special and a rebuild kit.