Grinding sound on decel

[TJ213]

I noticed that my car after sitting all summer is making a kind of grinding sound, also kind of like lose metal being rattled coming from the back of the car (I think the rear diff area) when I decelerate while in gear.

 

Any ideas on this? Its getting pretty loud (can definitely be heard when driving at highway speeds and windows up)

 

Also the same area clunks on any "spirited" shifting, if that's of any help.

 

 

I'm concerned that it may be something with the rear differential, and would like the forum's opinion of whether this may be a sign of a potential failure coming.