I own a 2005 F-150 XL that I was crawling around under the other day. I noticed what looked like a large steel encapsulated, rubber weight that was attached above the left rear shackle just in front of the bumper. Can anyone tell me what the heck this is for? NVH?

 

Vibration damper. Ford put them on certain models from the factory and others got them installed if the owner went in and complained about vibration.