Vehicle is a 3.0l V6 awd and has a shake that starts at 60km/h(35mph) the shake gets worst at 80km/h(50mph) and turns into a humming vibration at speeds above 80km/h. I have replaced shocks and struts, replaced u-joints and steady bearing, had tires rebalanced. All the steering and suspension components are tight and there is no audible clicking or grinding noises at any speed. Any ideas what the next course of action is?