In the golf swing, your shoulders need to go through full range of motion so that you can create a wide swing arc and clubhead speed. It is important to do shoulder exercises that take the shoulders through full range of motion and in different planes of motions.

Sit on an exercise ball, then roll down so only your shoulders are on the ball and arms bent at 90 degree angles.
Slowly rotate shoulders so forearms are perpendicular to the ground.
Return to start position and repeat motion for 8 to 10 repetitions.

This can be done with our without weights. This is a variation of the Standing Shoulder Rotators, but you are working muscles at a different angle.