I was able to tank heroic Culling on my warrior, I would say your block may be a bit low as paladins are as dependent on their shields as prot warriors are, but I can not see that being the only cause. I could only guess it is the loss of a pure healer.
http://www.wowarmory.com/character-s...rezim&n=Aimend

Are you using JoL or JoW, SoL or Sow?