...instead of altering the DcD's, you could add passives into each spec skill tree, that promote increased protection when played properly.

For example Balance Sages/Madness Sorcs are very static, they stay still a lot, during rotation, channeling stuff.. So, the passive for them should be; when standing still, all damage is reduced by... say... 15%. Nothing too crazy, but a consistent buff to survivability (stacking with other bonuses, when in effect), while staying still. If they move, the bonus is lost.

For Telekinetic Sages/Lightning Sorcs, they are the opposite. The procs with Force Speed allow them to stay very mobile, during the rotation. So their passive (gained in the skill tree at some point) should be: when moving, their Defense rating is boosted by 15% (additive to the base 10% class Defense). So as long as they stay on the move, they have 25% chance to avoid any attack. This bonus is lost the moment they stop.

That still leaves healing Sages/Sorcs, which are ok, in my opinion, since they can basically outheal most damage. Maybe buff their Self-Heal ability a bit more.