Should I rebalance my gear?

I tend to have problems with health in certain heroic instances, and I'm trying to figure out where the problem might be.

I'm currently at 27k unbuffed hitpoints, 22.5k armor, 19.3% dodge, 15.6% parry and 18% block. This adds up to 5 + 5.6 + 19.3 + 15.6 + 18 + 30 = 93.5% avoidance, including block, trinket can be popped for another 7% dodge. My current line of thinking is that I've over-spent on stamina, resulting in a lack of avoidance as most people here seem to go for the magic 102.4% first. I'm currently hunting for either the shoulder drop in Gun'Drak or Halls of Lightning, no luck there. Since no one has mats for the potency enchant anymore my weapon is unenchanted, as are my bracers as I expect to replace them soon.

A lot of people here also seem to have very high unbuffed block values (above 1500), so this could also be part of my problem as I'm currently at 1050 block value. Where do people primarily get their numbers from, extremely high strength or stacking up tons of block value items? I am assuming here that the trinket from VH as well as libram does not count as "unbuffed".

There is also the other side of the coin: My realm suffers severely from a lack of healers, meaning that the odds of getting a shoddy one is rather high. I have had days when I stick to a healer through a few instances and it's a walk in the park, while other days I struggle to stay alive with various healers. My guild has done 12/15 normal diff naxxramas (ranged tanks kept dying on horsemen), with me and a warrior alternating between main tanking. I was the hateful (hurtful?) strike soaker on patchwerk and he went down like a log without any survivability issues with either me or my co-tank with two healers in the raid, shaman and druid.

What sounds like the more likely scenario; that i've neglected avoidance and block value in favor of stamina and hitpoints, or that some of the healers I go with quite simply aren't doing their job?
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