Mostly left, actually. He hasn't had any symptoms in months, but in the spring around Easter time, if our foster puppy would jump on his hind end near the left side he'd yelp and jump up, and a 3 year old tripped and fell on his rear at Easter and he jumped up yelping then, too.
Then, late spring/early summer, we were playing at the park one night like usual, and we didn't exercise more than we had been, just a lot of dock diving, swimming, fetching the ball with the chuck-it, and he started slowing down a LOT towards the end, I noticed and stopped, he rested inside for about 5 minutes and then could barely walk, was limping so bad in his rear (both legs) that he couldn't even try to get onto the couch or into the car (I drive and SUV type, so it's higher off the ground) with his front legs off the ground putting weight onto the back without yelping (and he wouldn't try again after the first time). It lasted 4 days, I had to pick him up onto the couch, etc. and gave him some tramadol, which barely helped.
Tick borne diseases were ruled with a test before and after, and according to the vet it wasn't any kind of tendon/muscle/acl whatever from what he could see, but wouldn't be sure of an ACL tear/pull unless they did some sort of biopsy or something... which he didn't think was worth it right then unless it happened again, because the chance of BOTH going out at the same time gradually (no sudden incident) was very unlikely.
Had a dog chiropractor at a dock diving competition give him a rub down and she said his left side felt "different"... But he hasn't had any issues since, which is the weirdest part.
eta; oh, and we got rays of his back and legs/knees too, to make sure we weren't missing anything higher up and the doc said all was well.