I have a 99.5... When I bought mine, I thought it was a 99 (as it was listed in all the paperwork). As Kingman mentioned, the most obvious changes are the newer body panels (fenders are slightly revised, the front grille/headlights are the most obvious change, and the tailgate is reshaped a bit too.). Other changes include larger brake calipers in the front, and the shock mounts in the back have moved. I usually ask for parts for a 2000 to be sure.
The manufacture year is listed as Feb 2000 on the door jamb sticker.