I have a 2002 f-150 4x4, wondering if I have 28 or 31 spline axles, and where can I find gears for the front end?

 

02,
All F150s have 31 splines...at least the 8.8" rears do..
As for the reverse cut gears...most gear shops sell both. Randy's ring and pinion seems to have a good reputation.
Depending on what ratio you want...I found my 3.73's on ebay for a whopping $16 (+shipping) and they were brand new Ford gears... I'm sure others will have more places to get your rev cut gears.

 

The 1996-05 Ford Front 8.8 IFS axle is 28 Spline.

 

I fyour talking about the 9 3/4 rear axle they have 34 spline's.