A vibration under acceleration is rarely caused by just one thing. Sometimes worn universal joints are to blame; sometimes, it’s a driveshaft that’s fallen out of balance. In many cases, the root cause is buried further down the driveline than most drivers expect. The vibration itself is often felt before it’s understood, and that’s where things get tricky, in a way. As it turns out, the speed at which shaking appears, and whether it changes with speed or load, tells a technician quite a bit. What that vibration actually points to, though, still depends on getting underneath the vehicle.