Q: When replacing ball joints can you just replace the ball joints or do you have to replace the whole control arm?

The ball joint on this vehicle can be replaced separately from the lower control arm. However, on this vehicle it is also a known problem where the ball joint becomes loose inside the mounting in the lower control arm and will not hold tight any longer. This will require the entire lower control arm to be replaced. [https://www.yourmechanic.com/services/control-arm-assembly-replacement]

In order to know if just the ball joint or the whole complete lower control arm needs replaced, the lower ball joint will need to be disconnected from the steering knuckle and checked to see if it fits tight in the lower control arm. If it is loose, then replace control arm. If it fits tight then you will need to remove the ball joint from control arm and see if a new ball joint will fit tight in the control arm or control arm still will need to be replaced. This is why you were advised to replace the whole control arm.