Over time headlights develop a layer of oxidation that looks bad and reduces the effectiveness of your headlamps. Cleaning them and restoring them to their former glory is extremely easy.
Scuff marks looks terrible and are difficult to remove. A normal wash and wax won't take care of most scuff marks.
This guide will likely work on 4runners within a few years of 2001. All you have to do is make sure the part you buy is the correct part.