That's the steering arm that comes off the bottom of the steering gear box. It has a ball and socket that allows the linkages to turn and pivot. While they do get sloppy, the wheels can still be aligned. What they can't do is perfectly center the steering wheel.
Your truck is notorious for eating ball joints. They typically only last two years. I suspect your mechanic already checked them and would have said something. It's worth double-checking if no mention was made of them.