what does “jacking it in sandiego” mean?

It means “мᴀsтuʀʙᴀтing publically in San Diego.”

In 2012 a guy named Jason Russell made a video called KONY 2012 about an African warlord who was supposedly capturing children and forcing them to fight for him. There were questions about the accuracy of this video, but that is not important here. Shortly after the video was released Russell seemed to have some kind of mental breakdown. He showed up ɴuᴅε at a busy intersection in San Diego. People made videos of his actions which are generally described as мᴀsтuʀʙᴀтing, although he seems to have spent much of the time just doing ɴᴀκᴇᴅ calisthenics. That went viral with naughty bits pixelated.

Then South Park struck. Stan gets involved in some scheme which goes embarrassingly wrong and at the end of the episode decides there is nothing to do besides go “Jack it in San Deigo” which he does in a hilarious musical segment.

Jason Russell made a video called KONY 2012 about an African war