I wanted to make sure I posted this in the right spot.

My 2003 6spd has 74k miles and during hard acceleration makes a very loud vibration and grinding noise from underneath the rear, only under WOT. It's fine during normal driving.

 

hmmm, well i guess i'll just get a diag at Infinity and see what they say.

 

*update*

So it seems as if the cat is moving around violently and I may need a new tranny mount and possibly new motor mounts as well.
I should have specified that it is more of a huge vibration under heavy load right beneath the center console.
I actually hope this is the case because anything related to the driveshaft seems really expensive.