The problem with that suggestion Dezeral is that the dungeon finder tool does not allow shaman to select tank as they are not capable of that role. It goes by what bliz can say the class can be depending on the spec...shaman have 2 DPS specs and a healer. Paladins can be tank, dps or healer therefore are able to select any role (despite what their build is).

The only classes that can select tank in the dungeon finder are Druid, Paladin, Warrior and Death Knight.