A good exercise for overall shoulder health is the Y-T-W-L which is often done as a rehab/prehab exercise with light weight. The You Tube demo shows a bench used but a stability ball can be used as well.
You can start with just your hands and graduate to very light dumbells or anchor some light resistance tubes to the base of the bench.
The "W" and especially the "T" will hit the rear delts the best but all should help strengthen and support the rear delts.
http://www.youtube.com/watch?v=xck3d8LidME