According to some research online, it appears that on certain GM trucks the ball joints don't have zerk fittings to allow proper greasing. Thus the grease dries up or gets pushed out and lots of wear happens. Also allows dirt and moisture into the ball joint. Derp.
Edit: another catch appears to be the ball joint being riveted to the upper and lower control arm assembly. So if the balljoint is bad and you want GM part, you gotta buy the upper and lower. LOL