Two possible reasons come to mind. Do you see this in all models or just one?
In Shadows, is Use sun for shading enabled? Are the Light and Dark sliders crossed up? That is, is the light sider all the way to the left and the Dark slider all the way to the right?
The other possibility would involve the graphics card. What kind is it? Did this shading issue just start suddenly? It could be that Apple pushed an update to the graphics drivers in a recent update that caused this problem. If the Shadow settings don't fix it, go to Preferences>OpenGL and turn off Use Hardware Acceleration, Is there a change?