That would be the rear diff bushing, not a diffuser. It's a common issue. You have several options.

Live with it.
Replace w/ OEM
Replace with aftermarket urethane
Repair with epoxy (look in the DIY forum).

 

It’s normal happens to every g37, you can replace it with a stiffer rubber but if you are not having wheel hops or any vibrations you should be fine. Mine has blown also and I haven’t noticed a difference in the drive.

 

This is something I have on my list, but would this cause vibration at idle?

 

not that I know of but is your rpm dropping below 500 while in drive that is causing it? Or is it in every gear that it’s causing it? If it is in drive I would clean the throttle body and the MAF sensor it helped me.

 

At a stop light it usually idles around 600, sometimes dips to the low 500's. When driving the car is smooth as can be. I'll give the TB and MAF clean a shot and see what happens. Can I move the TB blades when the car is off? I've heard conflicting answers on this, and don't want to mess up the idle on the car

 

Yes you can move the flaps but you need to reprogram it it’s really easy.

 

We're getting off-topic. Let's get back to the diff bushing, please.

I also edited the title to accurately reflect the thread contents.