What to use to harden up a gravel driveway
What shape is the gravel?  Go look at railroad ballast, you want that stuff, aka "crushed stone", very jaggy and will interlock when tamped down. Very hard to shovel for that reason. You want a smaller size though.  

Any sort of round or half-round pea gravel type stuff has to go.  Get a loader, shovel it outta there, save it for aggregate for concrete. 

The difference is night and day.