1. drearier = darker
2. quivering = moving
In the first example, the road is being described as drearier creating an ominous and suspenseful mood. The best word to replace or describe drearier would be darker as it continues the ominous and suspenseful mood.
In the second sentence we know that quivering means moving, because of the context clues "to and fro". This means that whatever is going to and fro, or from, is moving.