2002 4.0 WJ, 185k on her, has a rear end rattle when idling at a stop, in drive with the brakes applied. Sometimes it does it on startup, but then it goes away. It doesn't do it when I'm driving, or when idling in park, only when in gear with the brakes applied.

It's confusing the hell out of me.

 

Look closely at your exhaust system. Source of many mysterious sounds 👌

 

Could be a broken catalytic convertor, its rattling around inside the canister.