Give them a "Dread Spikes" stance the reflects/drains 20% of damage taken instead of a revamped Shield Oath. (And you can even reduce their potencies so that they do less damage by themselves to keep balance)

Cons : Healers need to keep DRK at higher HP than other tanks for the drain to take effect after damage.
Pros : More damage when MT'ing with the reflected damage.

There, another stance for tanking that will make DRK as useful as the other tanks in 90% of the content. For the remaining 10%, it's fine for each tank to be better in a specific situation.