Why has no one made one? I figured the only reason would be because there would be too much sag in the driveshaft. I was thinking about doing this for my car.

 

We have one on our drag car, will put pics up later tonight.

 

Sweet. It looks like the AWD Motorsports one doesn't use either bracket...

 

I've been running the numbers and unless you can get a very stiff shaft, you are going to have too low of a low natural frequency.

I think you won't be any where near stiff enough to run a full length shaft with a metal driveshaft. A composite driveshaft might do it though.