Bryan, certainly, they can pop out and you woudn't hear a grind one, which means it's probably just starting to decline.
In a perfect world, the tranny shouldn't pop out of gear, EVER. So much for perfection.
As a rule, if the input has notable "wobble" when you try to deflect it, odds are great that the pocket bearing is hashed, which generally means it needs at least an input.
On a 4wd, it's really easy to see how out-of-tolerance it is, drain it (of course) then just stand it up on the rear flange with the output over a hole or something, then try to feel the play, or use a magnetic dial indicator to check it at the input.
Generally speaking, if you can FEEL play, it's likely pretty hacked already.
When lying horizontal, a seriously bad one will pivot, or "see-saw", meaning that when you push the nose of the input down, it'll push back.
I've lucked out and only had to re-shim a couple, but it's very rare.
Usually it needs at least a new input, but a re-bearing is always good insurance.
Put it this way, I NEVER buy a Getrag as a "good" tranny if the input wobbles or see-saws.
Mark.