Umm you'll never tank as a drg, never. There is really only 2 main tanks that people choose, and that is :

Ninja
Paladin

Ninja "blink tanks" which is basically, he takes no damage, assuming he/she is good at counting their shadows, unless of course theres an AoE(area of effect).

Paladin is all about defense and damage reduction, they take hits head on with a shield and heavy armor, they can also cure themselves to keep the focus of the monster on you.

warrior is a needed subjob for any tank simply because of provoke, but, sorry to tell you, dragoon simply won't ever be a tank, the healers would be healing you way too much, wasting their mp, causing huge downtime compared to a nin or pld.