I currently have the same character as you (stam dk tank). I am using defensive posture morphed with absorb magic because first its "passive" is good for a tank, i think you can block more damage(? i don't remember well). Then the morph allows you to heal when you're hit by a projectile, reducing the strain on the healer and allowing you to resist more in cases when for example the healer dies and he's being ressed. I don't use the other morph that stuns the enemy first because i prefer a heal than a stun, second because i don't think it works on bosses. and the trash mobs should die fairly quickly without your stun...
But I'm not an expert let's hope someone else answers.

Choosing Defensive Stance morph for reflect is wrong in PvE as a DK tank . If you want to reflect stuff , you can use Scales . Also , sometimes , you will want the Absorb Magic morph . It gives you a shield as big as half of your Max Health and heals you for about 15-20% of your Max Health when you absorb a spell . Here some examples :

Scales can reflect the attacks of the lady ghost boss in AA . In that fight , you never need to block . Just keep your wings up and just by doing that , you can do 10k+ easy DPS and have more stamina to support group .

Scales SHOULDN'T be used on certain bosses . Rakkhat in this case is a great example . The Gatling Gun attack , or Barrage , hits a lot . If you reflect them back , which reduces the damage you take to 0 , boss heals itself by a lot . Which is a thing you don't want to happen . In order to survive easier , you want the Absorb Magic morph .

There are other examples where you can use reflect/absorb but they don't matter much . These 2 are the most important ones . Hope this helps . Good luck !

Like Liofa said, Absorb Magic is a must have for endgame PvE tanking, but rakkrath is the only real must have. But hey if you want to tank stuff he is the thing you want to tank. I have never come to a place where I have wanted to use the other morph in PvE. In PvP sure it can be useful but the Dragon Scales are better there, and you can use both in combo to either healer yourself or damage the enemy. In PvP thou there is a 3rd thing you could use, Spell Wall, one of the morphs of the One-Handed and Shield Ultimate, I fricking love it in PvP. Far superior to Dragon Scale. While Dragon scale does deal 35% more damage when it reflect stuff back it can only reflect a total of 4 things and for 4 seconds. Spell wall can reflect all hits you take from projectiles for 8s. I have killed people with that skill in big zergs. Go as a tanky set up and stick your head out in front during a zerg and use that and people will aim at you and you will reflect it all for 8s straight. :P Its fun, try it. But for PvE go Absorb Magic.

i as well use absorb magic for the heal and relective scales for the reflect on my pvp dk tank

Thank you guys for the replies, now at least i see a rough info of the skills