Is it hard to change a ball joint on an 1998 ford explorer?
You’ll need a ball joint press such as this. http://www.sjdiscounttools.com/kdt3421.html And this might not come with the proper adapters. So the additional adapter set may be required.
Upper or lower? Depending on the condition of the control arm bushings and your plans for the vehicle, you might consider installing “loaded control arms”. That way you don’t have to press in ball joints and you get new control arm bushings. They don’t cost that much more than a couple of ball joints.