As a current 4th year at UCSD...
UCSD takes 4 residents a year. There are many Sub-I's but I would reccomend the Ortho Trauma rotation at the University Hospital as you will get to work with Dr. Schwartz, the PD who is an amazing teacher and student advocate. It is a tough rotation, and UCSD is closed to rotators unitl September so there tend to be quite a few rotators when the open the gates.
Previous research is part of an application, but I have yet to have an interview where I was asked more than cursory questions about my research projects or any ortho research that I am doing.
Finally, in applying for Sub-I's and residency positions via ERAS apply EARLY, i.e. submit the first day possible. I have seen many of my classmates torpedoed because the Sub-I's they wanted were filled or passed up for interviews because they were late in the game.
PM me if you still have questions.