What is the Spline on my 99 f150, open differential? I want to order a Powertrax No-Slip unit. Who has the best price?

 

If Your truck has 4.2L or 4.6L then it will have the 8.8" diff, which is 31 spline in a 99 F-150.

You might want to consider a Lock-Rite, which can be used in an open carrier. Lock-Rite is made by Dyneer Corp. (the manufactuers of Detroit Locker).