Is the softness of the driveway due to the water that pools up? I guess I'm having a hard time imagining a spongy sort of driveway.
If it has to do with water or moisture which is causing the driveway to be too soft, I would offer this suggestion. First of all, do you have a thick layer of gravel, or is the there a thin layer of gravel and then dirt or mud in your case?
I would look into getting some 3/8 lime rock or crushed gravel. Pack it down as best as you can. It will become more compact as you drive on it. I think that you could ask someone how much you actually need. After this packs down, you shouldn't have a soft surface, but rather more like concrete.