When did it start? What are the weather conditions when it happens? Are you using your parking break? FWIW this same thing happened in my Subaru during some heavy snow in Oregon. Turns out it was just my parking brake sticking. Every time I would reverse after I had been parked there would be a very loud thump from the rear along with a bunch of vehicle shake. Very scary the first few times it happened, but it went away after the weather warmed up. Also, if you can repeat this issue, then I would head to the dealer and let them sort it out. Looks like you still have a bunch of warranty left