Re: limited slip diff oil change..
Friction modifier is required to be added to Ford's Traction Lok LS rear end, and according to my manual, the Torsen LS rear end requires synthetic, but also specifies a bottle (around 4 oz) of the additive (which was about $5.00 CDN straight from the Ford dealer).
I haven't ever heard of friction modifier required for the front diff, and my manual doesn't list it as a requirement.
2003 Supercab Flareside
4 liter V6
4x4 FX4 Level I
5 speed Auto
4.10 LS Rear