The upper is one piece.  You must replace the control arm in order to
replace the ball joint..  The lower ball joint will separate from the lower
control arm though.

Am pretty sure the '98's are similar to my '96.  When I went to replace mine
there were two different types available.  One was like the OEM with the
integral ball joint (requires whole arm to be replaced), the other was from
Moog (I think) with a replaceable (and grease fitting equipped) joint.  The
arm with the replaceable joint was of course the more expensive of the two.
Did the job myself without too much trouble, although clearances are pretty
tight trying to get the arm mounting bolts out (can't remember if it was the
brake lines or the fuel lines that were in the way).  I did both sides,
including replacing the bushings on the adjustable arm (the non-adjustable
arm will come with new bushings installed, whereas the adjustable arm
doesn't include the part with the bushings, so to keep things equal I went
ahead and replaced the bushings on that side too.  Ended up using a ball
joint press to do it, with a makeshift sleeve made out of a pipe union to
push the bushings back in).  YMMV.

Steve