'02..FX4..SC...5.4L...There's some sort of canister attached to my stock muffler...about the size of a frozen orange juice can. It's not attached to anything else, and doesn't have any sensors or tubing or anything going to it. It just sorta hangs there. What is it?

 

I think it is a counter balance for vibration. I took mine off when I put on aftermarket exhaust. Does not seem to be needed, except I assume with the stock exhaust.

 

Yep, vibration dampener.