If the mounts get really bad, they can also affect the alignment of the driveshaft enough to throw it out of whack, which will give very severe vibrations at all speeds.

Did the shaking start suddenly? You could have an ignition problem, which will make the engine run out of balance (basically). I've seen a few spark plug wires go out and do this.

In order to solve the belt tensioner noise you will have to either shim the upper mounting point for the belt tensioner damper, or replace the damper.