Shaman's Role in Instance play

[Gurnsey]

I haven't seen too much discussion on Shaman here, and I figure it's probably because so many are Alliance, but I figure I'd ask.

My Shaman has been a Duo-er most of his life. In our Shaman/Druid team, I am sort of the 'Tank', and my job is to keep the enemy we are currently pounding on facing me, so that she, in Cat form, can backstab the life out of them. I don't neccessarily try to keep all of the mobs on me, as we can both take the hits pretty equally, so I'm not a traditional tank. We tend to trade off healing - if I need a heal, she heals me, and if she needs a heal, I heal her. We do very well - we are blasting through Desolace right now and having a great time exterminating the Burning Blade and rasslin' up Kodo.

Now, this all goes down the tubes when we want to group up. We are in our mid-30's now and we haven't done an Instance since a 4-player run of Wailing Caverns. We had a blast and we'd like to do it again, but...

What is my role as a Shaman in the group? I think I'd have trouble being a primary healer - shamans don't heal that well. I don't think I'd do too well being the Tank either - a Warrior or Bear Druid is certainly far better at that. Being a secondary healer doesn't usually take up too much time, and my raising and wipe-recovery I hope I never have to use (we had only one death in WC, from a mob spotting us across a chasm and running aaaaalll the way around to get to us, bringing every friend on the way), so what do I do? Am I damage? Should I use my Earthshock to interupt spellcasters, or does the extra aggro it causes make it not worth using? Are there many enemies who benifit from being Purged, or is that mostly a PVP tool? Which totems do people like to see (I almost never have a use for them in our duo)?

I haven't seen too many Shaman players on the boards here, but I'm sure many of you have grouped with them before at the mid to high levels. Can you help me find some direction?

Thanks!