What is this part of the exhaust?
 

For 10 years I've been putting up with a super loud & annoying rattle from the exhaust whenever at idle. Finally today I got under there and pounded on everything until I found it. The rattle is coming from inside this big bulging thing welded into the exhaust right where it comes out of the engine. What the heck is this? Is it the catalytic converter? I'm guessing this is not a cheap & easy repair, and I'm stuck with the rattle?

https://cimg5.ibsrv.net/gimg/www.jk-...92670f0255.jpg

From your picture -- Catalytic Coverter, that is an O2 sensor.... they will rattle when the substrate packing come loose or burns loose. Check the I-net for A/M cats which usually are less expensive than exact OE replacements.. Replacement will require exhaust system partial removal.