Darren,

To stretch your shoulders specific to golf, put your hands together as if you're gripping a club. Make a backswing and stop. Let go with your left hand and take your right hand as far back as you can while maintaing your golf posture and not tipping yourself over to get there. Now do it again, but this time take your right hand off and take your left as far back as you can, while maintaining good form.

Do the same on the follow through, but taking each hand of and taking that position as far as you can go.

This is a simple (no equipment) way to improve your shoulder flexibility specific to your golf swing.