I'll second the IS Boxer recommendation.

You get repeater regions, which make your healing so much easier.
Something like Vuhdo/Healbot for the frame + interpretation, or Grid/OpenRDX for the frame and Clique for the interpretation.
You can block the click on the local client, so they don't change targets, but send the click to a preconfigured group, such as the 'Healer' action target group.
You could have the priest in that group by default, and make a mapped key which can add or remove the shaman from that group on the fly, if you want the shammy to help on heals at times (or if the priest dies on a bad pull).

The Keymaps are incredibly flexible.
They control just about every aspect of how IS Boxer works.
You can target the active window, all windows but the active window, by slot (account), by toon name, by group (which you preconfigure) etc.
Keymaps can turn on and off other keymaps, which can be useful.

Mouse broadcasting is quick, easy and accurate.
For example, take the Warlock (Shadowfury), Mage (Blizzard) and Priest (Mass Dispel).
Create a couple Action Target Groups, called Broadcast 5 and Broadcast 9.
Let's say you put the Mage and Warlock into ATG-5 and the Priest into ATG-9.
You can set, "send next click" to ATG-5 as the target of your "5" key.
So when you press "5" that might be your 96969 macro on your Pally, your Click/Lightning Bolt on your shammy, but it is also Blizzard/Shadowfury on the Mage and Warlock.
And no matter what toon you are actively playing, you'll notice a target circle in their (Mage/Warlock) windows, which as you move the mouse on the main screen -- moves on their screens. Left click and their AoE's are targeted and mouse broadcasting turns off.

Well, amazing software, which I'd really recommend trying.
There is a 7 day trial, and a 14 day money back guarantee.
You could mess around with it for 3 weeks, without risk.

Check out my signature, for some of the things you can do with it.
Also check the guides/videos on www.isboxer.com