I’m not sure what the hell it is actually called, but just below the mufflers there is a bar about 3/4” around that basically connects the two sides of the subframe…..The rest of the undercarriage is spotless, but this one bar is rotted into three pieces. Does anybody know what it’s called and where I can get one? Please tell me it’s not an Infiniti only item…

 

You're probably talking about this piece, it's a very common component to rust to pieces, buy another one for your car but before you install it clean it up good and paint it with an undercarriage paint. Needs to be something flexible like urethane or something else, bed liner would probably work good too.

It's the lower front subframe crossmember connector. The FSM calls it "front cross bar" but you won't get very good search results

What happens is the factory piece flexes quite a bit and the paint cracks in many places then the metal underneath rusts away to nothing.

eBay is a good way to source a replacement, or check your local pick-n-pull. Make sure you get one for a 2007-2008 sedan, I think they were the same on RWD and AWD but the 2003-2006 looks similar but will not fit.

 

He said below the mufflers, I think he means in the rear of the car. I think he is talking about some kind of cross tie bar in the rear, just ahead of the mufflers, I think mine is bad also, but I don't know how much torque this actually has to resist.... However, I have been wrong before, on certain things, parts, on this car.

 

Ahh, didn't catch the muffler part of his post, probably just a stay brace, OP take a picture of exactly what the rusted component is.