x2
When a lot of people bench press, they don't know it, but they bring the bar down to their upper chest (almost their collar bone) and it creates enormous stress on the shoulders.
Try this:
Set up on a flat bench with just the bar. Pull your shoulder blades together as tight as you possibly can back into the bench and drive your feet into the floor causing a slight arch in your lower back. Have a spotter unrack the bar for you (so you don't lose tightness in your upper back and shoulder blades) and before you bring the bar down take a big breath of air into your stomach not your chest. Then when you bring the bar down, bring it somewhere between the region of below your nipple line to your upper abs expanding your stomach into the bar. This will do two things. Cut your ROM by about 30-40% while also causing less shoulder rotation during the movement. You may not be able to do much weight, but I'll bet after practicing you'll at least be able to bench press without problems. Also, try to make sure you're scapula and all three heads of your delts are proportionate in size and strength. If there are any imbalances it can cause a lot of pain. Eric Cressey has a couple of really good work outs for this sort of thing. Especially for this who have sustained injuries. I'd go to his website and check out his article section.
Here is a 10 minute video explaining how to bench like I said in pretty good detail...
http://www.youtube.com/watch?v=Dh3t6T-nqP0
Hope this helps...