Welcome! Now that you've asked for the "diving weight", you're officially an IH8MUD member. 
New LC100/LX470 owners aren't until they look under the truck and find that thing and ask about it, as every other user has done before 
Search for "what is this part?", "strange rubber pad" and similar thread titles
It seems to be a damping mass for filtering certain frame vibrations. Users that took it of have reported no noticeable difference. It makes an original paperweigth for the office desk.
Here's a related thread list




Search for "what is this part?", "strange rubber pad" and similar thread titles

Here's a related thread list