Keep in mind the stock swaybars on the FJ are smaller than the 4runner, so their aftermarket options may be equivalent to your stock ones. I read one calculation that stated our front bar is around 15% stiffer than their front bar. And yes both front and rear should be fully interchangeable, as long as you also order the correct size sway bar bushings.
Even before that you may want to try just upgrading your sway bar bushings to poly, that may be enough.
