I just did my lower and upper driver's side ball joints this weekend on my 1998 Explorer Sport. This walkthrough was great and I'd like to add a few things from my experience (since this is the #1 Googled search result on the topic of replacing a 98 Explorer's LBJ):
:exp: When I took the ABS Wheel Speed Sensor wire off my wheel hub, it broke. It was pretty well 'frozen' in it's home and a light pry with a flat-head screwdriver was enough to break the magnet/coil inside. When the ball joints were done, I now had an ABS light on
So, here's my helpful tip(s):
Remove the entire wheel hub, do not disconnect the ABS sensor (new wheel hub assemblies have a note in the box saying it is not a good idea to disconnect this, as it is calibrated in a certain way, I guess). The wheel hub is 3 15mm bolts, same as the caliper; easy peasy.
IF you remove the ABS sensor and break it, like I did, Autozone wants ~$150 for a new one (the whole ABS wire). However,
if you buy an entirely new wheel hub assembly, it also comes with that very wire for ~$115. Go that route! It's like save ~$30 and get a free wheel hub assembly too! My passenger's side wheel hub went about a year ago (wheel broke off on the highway, fun) so I was happy enough to replace the driver's side one as well.
:exp: I had to do my upper ball joint while I was doing the lower and getting it back in the steering knuckle was a pain in the ass. After much trial and error, I was successful by placing a large clamp stretched from the upper control arm to the sway bar to apply down downward pressure, holding the ball joint over the steering knuckle collar and then using a heavy ball-peen hammer to knock it down into the knuckle. Without the clamp applying slight downward pressure, the BJ would just bounce out of the collar on the knuckle after every hit from the hammer. A small amount of all purpose grease helped it go in as well.
Other than that, the upper ball joint was just as easy as the lower (though 3 times the price since it comes attached to the control arm, sad times) and in the end, it was the upper that was totally blown out and needed replacing, but the lower was in pretty good shape.
Good luck!