I'll repost my thoughts from another (
A Possible Bed Bounce/Vibration Temporary Fix with a Foam Damper) thread:
That's quite the idea...
Made me think of something else: has anybody who is experiencing the highway harmonics tried removing the spare tire? Perhaps Toyota's test mules did not have spare tires, and thus never experienced such a situation...
Just some random thoughts
Anybody want to try without the spare tire at all?