Crushjaw is located in the Cathedral of the Twin Gods on the planet Pandora. He can be found in the Eridium Sluice area near the end of the map. Fast Travel to the Cathedral, this way you are already halfway through the map, from there you want to make your way to the end of the map. When you jump down and are about to fast-travel the next area, turn around and go through the crack in the wall.
Crushjaw can also drop the Bone Shredder, a rare x4 vladof pistol with a high fire rate.