Have you looked into some Dynamat extreme or something similar (Roadkill mat) to help kill the sound/drone? 2 bulk boxes of Dynamat will do the entire floor pan, sections of the roof, and the inner door skins.

Right after that picture I was coming down the hill and hit a rock the size of a soda can that rolled onto the road. Thought for sure I put a hole in the pan, luckily it hit the skid plate and broke into pieces..it was loud inside! Theres lots of good canyon roads around LA, I grew up 2 blocks from Angeles Crest Highway.