It's an interesting idea. Be it's very extensive and probably cost a fair amount. I understand what achang1 is talking about where it would probably make more sense to invest the time and money on the back of the Jeep. Some sort of SUV boat tail.
One thing I have been looking at is a small kammback? off the back window. I've been noticing more of the newer SUV's out there with a little wing off the back. One thing I'm not sure about is if the airflow over the vehicle reattaches before the back window of the Jeep. That would be a big factor with this mod.
Another thing I've thought about is extending the rear bumper with some sort of a step with the right angle to keep the flow attached a little farther back.
Both of these would only be 6-10in long max. Just enough to get the effects going.