I just bought a 99 4.0 5speed 4x4 and was wondering how to tell what gears are in it I'm thinking it has 3.73s but not shure

 

There will be a label on the drivers door, rear edge, it will have an AXLE code, like "R6"

Look here for what code number means: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes

R6 = Limited Slip(L/S) 8.8", 3.73

Open means one wheel spins
L/S is what Ford calls semi-locking axles, Chevy calls it Posi-traction, both wheels spins

Front axles are pretty much all Open style, only the rear will be L/S.

 

**** I have 3.73s and an open diff
but thanks for the link anyways worked great