I think the pieces surrounding the rear side glass (including the "C" pillars on either side of the rear hatch) are fiberglass. Been a while since I had the interior out of my '91 S-blazer. If fiberglass, then you would need to fill the scratches, sand and paint.
A trick I have used on some plastic pieces (like, for example the area above the carpet on the tailgate trim piece) is to carefully apply heat with a lighter, propane torch or hot air gun. Scratches will often 'heal' themselves.
HTH