FJ's manufactered after november already have the mod - although there are some specific conditions that apply to the mfg version of the mod (i think it will only work under 3-4mph).
The mod lets you engage ATRAK while your rear locker is engaged. (Normally, Rear Locker takes precidence over ATRAK if both are turned on).
The purpose is to give you additional front wheel drive traction while your rear is locked.
Toyota will not do this mod for you. I am not aware of the warranty issues of doing it either - maybe someone else can address that.
The mod looks pretty easy to do. It doesnt involve cutting any wires so it's easily reversable.
This is the post on fjcruiserformus.com on how to do the mod. I'm going to be doing the mod myself so maybe we can have a mod party.