The lower ball joint is pressed into the knuckle and is replaceable if you have a press to do the job. Harbor Freight sells the press kit for about $50. Lower ball joints sell for about $50 - $60 at Advance Auto Parts.
The upper ball joint, the spindle nut, tie rod end will also need to be disconnected to accomplish this.
If you decide to do this job yourself, you will need a vice to hold the press and a buddy to hold the knuckle. Also, make sure you add some grease to the ball joint before reinstalling the knuckle. They never put enough grease in them. You don't need a lot but more than what they put in.