Probably the EASIEST Upgrade for your FJ:
As you may know, FJ Cruisers equipped with a Rear Locking Differential, but NOT the A-Trac button can add A-Trac functions VERY easily:

First, obtain your A-Trac button (yes, it HAS to be that button):
-TRD Source
-Your Local Dealer
-Online

Second, take a SMALL screwdriver ( I used the smallest head on my Leatherman) and push it in the top of the BLANK right next to the RR Diff Lock button. Pry up slowly and the button will pop out.

Unplug the connector from the blank. Plug the connector in to your A-Trac button. Push the A-Trac button iinto the hole unti you hear it click in.

You now have A-Trac. Congratulations!

There's another great writeup here  

In the future, we'll add MUCH more detailed and exciting tech articles. Check back :)