there is a vacuum motor on the front axle that engages and disengages the passenger side axle shaft from the rest of the axle.

here is a pic



if you look under the diff on the left side, that is the collar that connects the two piece axle when it is engaged, allowing the passenger side axle to become one piece and get power to the passenger side wheel.

 

Check out this link
http://www.ford-trucks.com/forums/92...97-2003-a.html

When you turn on 4wd, it disengages the vacuum lines and engages the diff, connecting it to your front axles.

 

makememad94,
I have been running 4.10:1 in my 2002 4.6 4x4 F150 for a little over 4 years now. I also have a Truetrac in the rear diff. I currently have 285/75-17 Nitto terra Graplers with the 2002-2003 factory FX4 wheels.
I really love this combo, and i frequent backcountry fire roads here in the mountains of Western Washington. The truetrac helps out pretty good, and like str8t six suggested, the application of brakes while applying throttle works great with the truetrac when one of the wheels in the rear is off the ground with no contact. The only thing i have to say that could be bad, is the rear can get sideways really easy with this combo- just dont let the wife drive it.
I get pretty decent mileage, probally 14mpg average and up to 16-17 highway with this combo.
Hope this info helps

 

Ok I have a better understanding of how it works now. Thank you. I can't say I like the design but as long as it works haha

 

Those sound like pretty good numbers to me. I'm pretty dead set on the 4.10s and true trac now. I'm glad to hear a good personal experience