Judith,
We live on a girl scout camp, lots of dirt/gravel roads. We have found a device called a TR3 really helps with the road maintenance (better than blade, box blade, the only other tractor tools we have that MIGHT work). It takes several passes, some to tear it up, more to smooth it, but that is what it takes.
It isn't cheap, but we use it to help keep the horse arenas in shape too.
You might see if a local farmer (or county road maintenance crew) might come smooth it for you. It should last a couple of years if done right.
I understand the 'washboard' effect happening!
Take care! ... Jack
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