Some thoughts:
*the movable portion of the tail fits inside the fixed,upper portion.
*Anything which 'rides' on the ground (skid,wheel,etc.) would need to castor,with 2-degrees of freedom,or else in a sweeping
contact,the diffuser will be forced laterally,and will jam,and want to break out of the upper units perimeter enclosure.
*Ideally,you'd disable the diffuser until,say,45-mph,when on the open road,with no chance of any contact at all;before the lowering mechanism could be enabled.
*Automakers are relying on air 'suspensions' for active aero.
*A simple ram,with reversing valve would allow air to be applied to either side of the piston.
*A cruise-control logic unit and air solenoids would be enough to control the ram.