Even if this is not the last segment on your ticket, mini micro-hops (time in the air can be as little as 15 minutes) are ones which almost any AA agent will cancel out for you at LAX and preserve the remainder of your ticket.

If you call and ask, you will get the entire speech about paying the change fee + fare difference, but ask at an AC or counter and almost any agent will do this for you. If you hit the one stickler, ask another. In the end, you are no worse for having tried.

Rent a car, take a bus, taxi, Uber or whatever. It will take you <2 hours and you will be in San Diego 4 hours before your onward flight even departs LAX. As others note, you must claim your luggage at Customs at LAX anyway.