I did similar when building my driveway as there was a lot of fill dirt used after removing small, buried boulders. Some areas were very soft and even my Jeep could get stuck. I laid 1.5" of slag over the whole driveway and end of problem. I think they interlocked well enough to not require packing down into the soft stuff. It actually worked far better than expected.
Good advice from all: filling in mud holes in on my list for the Fall if dry enough...too much to do this summer.... Around here 'bank run gravel' is cheapest fill to get delivered, and I suppose it would work OK, but not as good as 'crushed stone'.