I don't think Living Dead is the most powerful Tank invulnerability at all, Walking Dead is the "actuall effect" for Living Dead skill and it's has 10 seconds, 17~18 secs it's just a illusion for the skill total time and how it's works.

Living Dead do not increase your defense or anything else, so if the healers are going to ignore their Heal to activate Walking Dead by purpose the damage you take under Living Dead is not reduced at all. Any Tank can do that, Healers can completly ignore their Heal and when the Tank is almost dying they cast their skill and become invulnerable as well, the difference is that DRK don't have to wait their HP goes under 10~15% for that kind of situation and can cast it a bit early, the amount of damage you take in that process by not get healing is the same for any Tank.

Living Dead is to me the worst Tank invulnerability by far, there's some situations like Big Pulls that's basically a guaranteed wipe for SCH/AST because it's pretty hard or impossible to heal 100% in time. While Holmgang don't kill you if you're not 100% healed, Superbolide gives you damage invulnerability for 7 secs reducing your HP to 1 ( don't use that with Full HP as i've seen many GNBs doing, best time is the lowest HP possible so 1HP effect become almost irrelevant ), and Hollowed Ground is the best by far, Damage invulnerability by 10 secs without any side-effects like chance to die if you're not full healed in time ( DRK ), immobilized without be able to walk ( WAR) or HP Burning cost reducing to 1HP ( GNB ).

My sugestion:

Living Dead: everything still the same, Walking Dead killing you if not full Healed in time, that makes the skill unique, but i would add a new effect on that, and should be something like "Additional Effect under Walking Dead Status: increase HP recovery via healing actions on self by 200%, ensuring Critical Healing effect". That way Healing wouldn't be a issue anymore and the skill remains the same.