Do I need to replace both sides at the same time?

Best practice is to do both sides together so suspension geometry stays balanced. If one arm is worn, the other is likely close behind. Your F-150 will handle much better with matched components on both sides.

Will this work with my current ball joints, or do I need new ones?

This control arm comes as an assembly. If your original ball joints are shot, you may want fresh ones since you're already in there — old joints won't benefit from a new arm. Inspect them during install to decide.

Do I need to get an alignment after installing this?

Yes. Even though this uses factory mounting points, suspension work always needs a full four-wheel alignment afterward to ensure proper toe, camber, and caster specs. Your truck will pull or wear tires fast without it.