That is the setup I am working on atm. Pally tank,Druid healer and 3 Shaman dps. The Pally, Druid and Shaman are 70 and I decided to try and 5 box by adding two shammies. But my advice would be not to go druid unless you want it for other reasons outside of 5 boxing. I love my Druid but healing a 5 box is a pita because your "main" heals are hots which must be constantly stacked/reapplied. I would suggest another shammy for ease of leveling or a priest/paladin over a druid. When 5 boxing, you don't want to micro manage your dps or heals.