What is this ground for? Is it needed? It was attached to the stock exhaust before I replaced it with a Greddy Evo2. Here is a pic:

 

dont know but my guess is for the o2 sensors..

 

Its in the far back by the muffler. It is attached to the rear subframe.

 

066MT,
...just my $.02 but the only thing I'm sure of is that it was put there for a reason..maybe to bridge the ground circuit, lowering the resistance, to the rear chassis. I'd find a way to get it back on if I were you. Infiniti wouldn't have added the cost and labor to install this if they didn't feel it was needed.