If I wanted to keep the grassy center, here's what I would do.
Find a dump truck driver who has done it or is willing to try: Place a piece of RR tie or concrete block against the center of the tailgate before loading. When tailgate-spreading the load, it will come out in a stream on each side but not the center, filling the ruts but not covering the grass.
Grade and maintain the road with a road-width landscape rake with the center teeth removed, and with gauge wheels.
Bruce
So basically you feel just add gravel back between the center grass to level it all out. I have no issues with this at all and it's actually easier on me. I'm wondering since I have a nice super compact gravel base what about using crushed limestone or something similar that packs hard.