GM's Aero 2000 'Epcot' car had an active rear valance/diffuser.In the video at Epcot Center,in the GM exhibit,you can see the rear panel lower to a more favorable angle when on the highway.
GM's the only company to my knowledge to do it so far.
It's worth knowing,as you get your 'RUBICON' drivabilty,and also some slick open road performance.
Here's a link
1982 GM Aero 2000 Concept Boldride.com - Pictures, Wallpapers