In slippery conditions, if I induce a lot of wheel spin, I get this nasty banging sound and feel from the rear end. I doubt it is wheel hop because it did it on wet grass while trying to sling the rear around-and I doubt seriously wet grass could cause wheel hop. Any ideas?

 

Rear diff bushing

 

Never figured out which trim level you had but if your car is equipped with VDC (or even TCS to some degree), there’s a slight possibility that VDC is cycling on/off trying to catch and level the car.

But yes, it could also be mechanical in nature (as Trav pointed out)

 

Z's are notorious for this. Assuming your shocks are in good condition, you have to go through these and narrow it down:

Subframe bushings
Diff bushings
Diff bolt (Z's have 1 bolt holding the diff to the rear subframe. Sometimes this shears/gets destroyed. I believe the FX35 is the same design but the bolt is noticeably bigger/stronger.

I'd install a diff brace as well. I have a Bell Raceworks unit and am very happy with it.

Apparently the one way to nuke said problem out of orbit is with solid bushings in the back (subframe and diff). I'll be taking out my Whitelines and doing that next year as I also still have some axle hop (that's what your problem is called btw) when I launch in first gear.

 

Feel bad for that grass

 

Came in here thinking OP wrecked his car in a parking lot drifting accident! -_-