Not sure if any of the 3rd gen guys have made there way here to explain this yet, but I figure I'll throw my 2c in here as a 3rd gen owner and a mechanical engineer...
The 3rd gen LBJ is a poor design. Toyota fixed the flaw in the 4th gen design.
The 3rd gen LBJ, when the vehicle is just sitting, is under tension, meaning the suspension preload is oriented such that it's pulling the ball out of the cup. When driving, every little bump and articulation of the suspension just pulls the ball out of the cup even harder. When the ball eventually wears and fails, the suspension yanks it out of the cup and the wheel essentially falls off (it's still connected by the UBJ and the brake line, but the damage has been done and those won't hold it on in any meaningful way).
The 4th gen LBJ is essentially upside down from the 3rd gen. Suspension preload presses the ball into the up. So if the ball and cup wear out to the point that the ball could pull out of the cup, the suspension is actually holding the joint together. That means no wheel falling off. You still need to check it to see when the fit has loosened from wear and a separation becomes more possible, but no catastrophic failure like the 3rd gen.
The prevailing theory here is that to account for the design flaw (I call it a design flaw because it is a flawed design), Toyota used very tightly toleranced parts in the LBJ, and probably used some high strength metal alloys to keep the LBJ from failing prematurely. The OEM parts appears to be very good, and I haven't seen anyone report an OEM failure under the 90k mile replacement interval for the LBJ's. For aftermarket LBJ's, most commonly Moog, they can't afford to put in the materials and process that Toyota did, so they use cheaper stuff, and as a result, the LBJ's fail in a little as 10k miles (quite a few reported failing under 30k miles).
Sorry if that's repeated information, but in searching for another thread I found a couple 4th gen threads asking about this so I figured I'd post it up.