Mysticz is right of course. What do you plan on doing with your 4Runner? I have ARB lockers front and rear, the large capacity ARB compressor and I've also re-geared front and rear at the same time. Depending on your intended use, it could be invaluable, or money best spent elsewhere. As far as rear only, again...depends on intended use. Give a little more info and I'm sure you'll get more useful feedback.
For me, money extremely well spent. I've only needed to use them a few times (and mostly rear only) but when you need them they will make you happy you spent the money.