First off, I've been playing since about 3 months before Vanilla-Naxx was released so I am not new to the game, or to tanking.
Lately I've been doing alot more raiding with my guild, we have been clearing Naxx 10 every week since we formed near 2 months ago, I am usually OT for my guild except on some fights, my gear isnt that bad but I know its not great, but I am constantly being told that I am nearly un-healable on hard hitting bosses, I need to know if theres something I did wrong with my gear or if its just my healers.
Care to link the other tank? Your stats are beyond what I would consider low end for naxx. I'm curious to see if your other tank covers for deficient healing.
Spec wise you will probably find a lot more use in taking 2 from shield specialization and throwing it into gag order. The silence is nice but the 10% shield slam damage is a must from a threat standpoint (and you're losing block rating, big deal). Also If you're going to spec improved disciplines, glyph for shield wall, taking it down to a 2 min cooldown is really nice.
None of the above changes are going to make or break it for you. Knowing what your other tank is like, or seeing some logs might help though.
While your gear is at entry level for naxx 10, I would keep doing heroics looking for upgrades. By using 3x +27def dragons eye gems, you are cutting out almost 1500 extra HP. That could be the difference between the healers having some leeway on you, and them struggling to keep you topped off because you get 2-shotted.
No clue why your healers would have an easier time healing him than you. Some bosses you can expect the damage and blow shield wall or some other cooldown but nothing in naxx hits so hard that they wouldn't be able to keep you up.