What is this block between the bed and frame

While under my truck, I just noticed this rubber block between the frame and bed only on the drivers side of my Gen 1. It's all the way back to the end of the frame and sits loose and able to be moved around even though the bolts are tight. What is this and what does it do? I don't know why I've never noticed it before and why is it only on one side?

It's a harmonic damper thingamajig.

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Keeps vibrations down in frame from what I'm told.

So there isn't supposed to be one on each side and it's alright for it to be so loose?

Its supposed to wobble, thats what helps absorb the vibration from the frame.

I think it was a bigger problem on the trucks with the longer 6.5' bed. It was definately an issue on my old 2006 FX4.

There is supposed to be only one on the drivers side.