The "rockcrusher" moniker is/was applied to the Muncie M-22 car transmission. It was so names because of it's straighter cut gears that had difficulty meshing even with the syncros in good condition. Helical gears mesh better and don't make as much noise as striaght gears. It also had a tendency to whine going down the road. If it is a truck 4-speed with granny low, it is not a rockcrusher.
The 4-speed you have is likely the sm420.
