I can't tell from the picture if it's the spindle that's cracked or the ball joint. No matter where you order a spindle it will most likely come bare. Meaning you will need to press the bearing, seals, etc in it.. If the bearing is fine then use it, otherwise replace. I recommend buying new seals though but watch out as they're rather costly. There's no problem buying used, say off of someone parting out their 4runner. This could save you some money and time as it's purely a swap and go kind of job. If you're handy with a wrench it's not that hard.
I don't think aftermarket balljoints caused the crack. I haven't seen too many instances of this though so it really could be anything. I think it's just a faulty spindle. It could have happened when replacing your ball joints last and you never noticed as working (banging, wrenching, pulling) may have caused some extended stress on it and after a little time, it cracked without you being aware of it. Who knows.
If the spindle is cracked, get a replacement spindle asap. Otherwise you can end up like many before you on the side of the road with your front end in the ground and your tire 50ft. down the road. No fun. As far as aftermarket upper control arms (what the guy above me meant with uniball uppers), I don't think they're needed in this situation.
Good luck
:llama: