The free dictionary gives this definition for rip on:
give someone a hard time; to hassle someone
and this one for pick on:
to harass or bother someone or something, usually unfairly
It sounds pretty much the same to me, but maybe there's a slight difference I don't catch?
Picking on someone implies merely bullying -- unfairly making fun of or harrassing someone because of something they can't help, such as their physical appearance or mannerisms. An example would be a short person being harrassed because of their physical size, or a person with a mole on their face being harrassed or made fun of because of the disfigurement. Or because of their religion or nationality, etc.
Ripping on someone involves severely criticizing their actions or their political positions, or even their work, such as your boss ripping on you for your bad job performance. Ripping may be fair or unfair, but it is generally directed at actions or attitudes.
"Ripping On" is a verbal exchange or a mocking behavior of someone, deriding them for the way they act. Specifically, to put down or belittle their way of acting.
"Picking On" someone means to bully or harass someone. It can be that the person picking on someone else chooses to do so for the reason that they see fault in the way a person acts or behaves, but is not always the case.
"Picking On" can also include physical violence, where "Ripping On" usually does not.
"Picked on" can also imply something ongoing, or just a general climate of bullying or of being outcast. "I was picked on in school" could mean all through my school years, I was constantly the victim of bullying. (It could also mean "I was picked on in school today.") "Ripped on" is usually confined to describing a particular occurrence.
