The Pally is an exceptionally good single target healer.
They can Beacon to heal a second target.
And through a Tier 1 talent, heal themself a decent amount too.
They don't have a lot of tools for AoE healing, so will be a bit challenged anytime they need to heal the group.
Still, the Priest can bubble the party; or you could run him/her as Disc which would help with heals (Smite can heal the lowest nearby party/raid member, via talents).



I would personally run the Pally as the Tank.
Both a Priest and Shaman make excellent healers; take your pick.
I'd likely go with Shaman Resto, because Earth Shield + Riptide is fairly mindless healing and strong enough for most encounters.
Then the Priest can go dual-spec Shadow or Disc, depending if you want offense or support heals/shields.

Run the DK as Frost, following Mosg2's DK Manifesto; you'd have to play with CTM/IWT, but with DK/3x Ret you probably already were.
Warlock could be any spec; I like Destro for the ranged stun and replenishment.