If you soak the immediate area with some water it will soften up the soil. Not on a mail box post, but I have used 1/2" copper fastened to my hose as a water drill to get moisture deep into the soil. Should be easier to push back in place, no sledge hammer, except for the jack-a$$ that damaged it.
Note, talk to local police as often more than one gets hit and in today's time possible a camera shot has already identified them.
Bud