Just because the hubs are in the open position doesn't necessarily mean they are not still locked in. Under normal use you can bend the ear tabs on one of the bands inside. These tabs are very wimpy and can bend and are what pull the engagement gears apart and hold the hub open. If they are even bent a little the hubs will not fully disengage and cause the front driveshaft to spin. This will cause a high speed vibration/shake. Also old worn steering and suspension components will also create a high speed vibration/shake, and as stated before worn out driveshafts and worn or loose pinion and pinion bearings will cause this too.
