OK, I spent about 20 min this AM typing up a response to this and it got lost in the ether and I got frustrated. Now I'm going to try again but please realize that if the tone of the response seems terse, it's not my intention... just happens that way because of what happened this AM.

Rear delt exercises have to be done with caution, the elbow whould not go past the line of the torso, this adds stress to the shoulder and may actually make the pain worse. Here's a visual example of that form mistake. Here's an example of a rear deltoid exercise where the person doing the exercise is not mving his arm too far back. I really like exercises that use your own body weight like this one. What I like about this is that the weight is effectively less as you get into the form danger zone. You can easily do this with webbing or rope, you don't need anything fancy. This is also a good option using cables.

The other things to keep in mind is that you need to do pec stretches and specific rotator cuff strengthening is also important.

Hope that helps.