Asked a similar question a couple of weeks back, didn't really get much of an answer.
I was looking at mine under the truck, and noticed that there was a "crack" in the material over the u-joint cap (uses a small pressed in metal cap I guess) and the axleshaft, which is apparently paint, because it's hard.
I suspect this is an indication that they are going bad. The one axleshaft I've seen before that had a bad joint in it was also missing this, like the u-joint caps start to walk out of the axleshaft or something. Just looked it up in the parts manual, they don't break it down any further than "u-joint repair kit".