-Soft tissue work
--Tennis or lacrosse ball for posterior shoulder joint (infraspiatus, teres minor esp. . .)
--Stand up straight, push your shoulder as far forward as possible. Maintain that position and feel for the front of your shoulder blade in your arm pit. Here's a visual of what you're looking for: http://z.about.com/d/exercise/1/0/G/k/subscapularis.jpg Massage that with your thumb. It may be painful, but that's because that muscle works hard all the time, and it has developed scar tissue as a result.
--Shoulder girdle (rhomboids with duct taped tennis balls, sure this is mentioned somewhere around here in an article)
-Balancing out the foo-foo stuff
--You've already discussed imbalances in external rotation, this is an indication of weakness of muscles such as infraspinatus and teres minor, which work hard to depress the head of the humerus into the glenoid fossa when you are benching. You see, pectoralis major has a tendency to produce a 'gross' type of motion on the humerus, which sort of acts to pull the upper arm bone out of its socket. An imbalance of activation, strength, and/or length of these 'external rotators' can cause pec major to get over worked and more hypertrophyed (as you've noted).
-Strengthening the foo-foo stuff
--Start working on prone internal rotations on the side where your pec is larger. Your pec may be working harder than it should be, which could eventually lead to strains and all kinds of lame stuff. Getting subscapularis (the muscle worked when you do prone internal rotations) stronger will take some of the load off of pec major and keep that shoulder joint going hard.
-Understand this takes time. . .
--You've mentioned this is something that has been going on for some time. It will also take some time to fix.
If I could see you in person I'd be able to do specific tests to figure out exactly what the problem is. Over the internet, I can tell that you need to do soft tissue work for just about everything and specific strengthening work for your rotator cuff. So take the day after your pressing day and spend time working on cable external rotations, prone internal rotations, and scapular stability stuff. It'll take you 15 minutes to do that stuff, and if you spend 5 minutes a day on the tennis ball and foam roller and stay away from stupid shit in the gym, this issue will resolve.
Also, I know you don't want to make huge changes in your training, but it may be a good idea to take the weight down and REALLY iron out any form discrepancies you may have. If you are leaning more to one side on the bench, this can reinforce a triceps/pec/intrinsic shoulder imbalance you have.
Listen to your body, man. If you go into the gym and your warm ups are hurting. . . That should tell you something.
Great luck man, I know you'll resolve this!