Jason
Not having a jag rear end ( Live axle 9" Ford ) I'm wondering if you could add a big arc 90 degree sheet just under each seat rear floor area at the very back, that turned up into the area behind the seat back and bring air over your diff centre and the brakes that way?
Basically an air scoop upside down with air escaping upwards onto the diff centre each side.
The sheet would only need to be about 15 - 20 mm below the floor if it was wide - say the width of the floor below each seat.
It would drag a lot of air up from under the car. Don't think it would cause any lift!! The floor below the fuel tank would cause more uplift!!
Make sense?
Bernie