I know little to nothing about cars or trucks. My '05 Chevy Colorado is "shaking" and "bumping" hard, especially when not moving, in reverse and under 15 mph. It resembles the shaking a car makes right when it runs out of gas. Once I get it up about 25mph the problem basically disappears. I'm hoping it's nothing too serious, but am in the market for a new truck anyway, so this might just make me go for it. Any ideas or suggestions? Thanks fellas!