Longevity of our balljoints is hard to figure out. Some guys have much better luck then others. Tire size, road conditions, and driving style probably have a lot more to do with it than just mileage. I check mine frequently, and wouldn't be surprised if they didn't last longer than 15000 miles when offroading frequently. The bottom line is if they are loose, replace them. It's been my experience that if you ignore a couple of sloppy suspension/steering components for very long, you have a better chance of everything else wearing out faster. Keeping your front end tight equals a much more enjoyable truck to drive.
If I was you, I would check them myself. Taking someones word for it is always hard when seeing the repair estimate. I posted how you can do a basic front end check in the 3rd Gen forum yesterday. It's easy, and worth doing as basic maintenance.
KP