you also have to work on your flexibility and muscle balance as well as address the cause of the pain. this is a rehab so use weigth training as a part of the strategy. sage is right in saying you should strengthen your rotator cuffs, particularlly the external rotators, but you also have to strengthen other muscle groups that are weak. you state that your chest pulls your shoulders forward. this means that your upper back is also out of balance with your chest. to keep the upper back in balance, you have to do a similar volume of horizontal rowing as your chest exercises, if it is out of balance, then you have to do more for the weak part and give it priority, ie: do it first in the workout, do it first in the week, do more volume than the strong part, etc. the same goes for the balance between shoulders and vertical back exercises, there must be a balance.
so what can be done to help the situation in the weight room? well first off the external rotators most likely need work. do them first in any upper body workout. second, stretch the chest and work the upper back, both with rows and shrugs. also when you deadlift, keep your shoulder blades pulled together and down from the bottom all the way up. quite obviously you will not be able to lift as much weight as this will not hold out at your normal weight. it is a weak point which must be overcome. you may find that if you ease up on chest work and concentrate on weak points, they will develop quickly and then you will be able to hit them hard again to get to the next level.
the concept of muscle groups balancing each other and having good flexibility are beneficial to all of us, whether we are bodybuilders or other athletes who use weight training to improve performance or even those of us who workout just to keep in shape. incidently, the concepts in this post are taken from articles i got from louie simmons of westside barbell fame from the elitefitness web site:
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hopefully this satisfies YJ since they are not from t-rag or trash mag or whatever he calls it, although ian king has similar ideas (JUST KIDDING YJ

).
cheers, pete
PS: hey gmonkey, if you give us a little more detail, we can give better perscriptions for dealing with the problem