You can dislocate a rib but I have never heard of any one getting it from good mornings.
In fact last year I was horse playing at work and gave a guy a bear hug.
I dislocated his rib and it cost me a few hundred bucks to take him to hospital, get a chest X-ray and medication for the miserable bastard who tried to milk it for everything he could.
It healed with time.
If it aint right yet go see some one!