Before dismantling anything, I'd check actual transmission temperatures with a handheld infrared thermometer....compare port and starboard and then you'll know for sure. No need to do a lot of work before you first determine the cause.

My Allison M20's run at about 185 degrees....I added in helm temperature gauges to each, as I had a transmisssion overheat many years ago on another boat...it's cheap and provides regular feedback from an expensive component of your drivetrain.