For heroics you only need 535 Defense to be uncrit. You're sitting at 548 at the moment. Re-gem your DEF gems into STR, as it will raise your block and get your DPS/reduction up a little bit. (I'm re-gemming to avoidance now that the DPS I run with is better geared)
Also, re-gem your helm to the 21def +5% BC, and re-enchant your cloak to armor. 16def is overkill, You may want to buy a tattered castle drape off the AH and enchant armor on that, since 16d isn't a cheap enchant and you may need it later on.
We kind of have to carry DPS a little when first starting heroics, the things above should help out quite a bit. Don't worry about having 21-23k health when starting out as a pally tank. I had maybe 21900 when I started doing heroics, but it didn't matter.
Also, try and get an essence of gossamer as soon as you can. It makes a noticable difference.
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