This is kind of intriguing to me, as the last character I played was the tank for the exact same campaign. I also played a dwarven fighter (although I opted for the Anointed Knight Prestige Class instead of Dwarven Defender). My sole goal was to be able to soak up hideous amounts of damage and keep on ticking (my prior character had met a rather untimely end due to lower hp). I did go with the high AC route and it actually worked pretty well for me. I was able to take the abuse in pretty much every combat
I'm not sure if you've got Races of Stone or not, but it is a pretty good book if you're after a Dwarven Fighter type.
There are two feats in ROS which improve your ability to wear heavy armor (+1 AC and a lower Armor Check Penalty). See about getting those. The Dwarven Fighter Racial Substitution Class has an equivalent abilty at 8th level, for a total of +3 AC and a chance of actually successfully treading water if you have all three.
Ditch the mithral full plate in favor of a suit of Admantine Battle Plate (ROS again) if you can manage it. You'll need another feat here too. A slightly higher AC, plus a - DR. Even if you don't get this, I recommend the admantine plate armor over the mithral.
Start stacking up the little bonuses!!!! I can't emphasize this enough.
Your shopping list needs to include:
Amulet of Natural Armor
Ring of Protection
Ioun Stone
A +2 for each of the first two is fairly inexpensive and together they'll up your AC another 5 points.
Definitely use Combat Expertise and consider combining it with fighting defensively. The big trick here is figure out how much to use and when to use it. You goal should be have an AC 20 points higher than your opponent's Attack Bonus and no more. An AC of 34 (get the little items I mentioned above) can shift as high as 41 using combat expertise and fighting defensively. What is important to realize is that you get the most benefit from shifting the required attack roll to hit you from the 15+ range to 20. Shifting up the lower end required attack rolls to the mid-range has less of an effect than shifting a high roll higher. You want to be in that high roll range.
Let me illustrate my point with some examples:
Attack Bonus +18 and averages 40 points of damage/round
AC 29: Hits 50% of the time, averages 20 points/round
AC 34: Hits 25% of the time, averages 10 points/round
AC 39: This 5% of the time, averages 2 point/round
At AC 29, you'll probably last about 7-8 rounds against this foe.
At AC 34, it goes up to 14-15 rounds.
At AC 39, it is up to 70-80 rounds.
You'll note that the move from an 11+ to 16+ attack roll needed cuts the average damage in half between the two numbers. Moving from 16+ to 20 cuts the average damage by 80% (or 90% if you're comparing between 11+ and 20). Big difference in how long you can last.
Example 2:
Attack Bonus +30 and 40 points of damage/round
AC 29: Hits 95% of the time, averages 38 points/round = 4 rounds
AC 34: Hits 85% of the time, averages: 34 points/round = 5 rounds
AC 39: Hits 60% of the time, averages 24 points/round = 6-7 rounds
You'll note that the AC boosts are less effective here, although they do have an impact.
There is essentially an AC point you want to be above for AC to really be effective at cutting down the amount of damage you take.
Just my thoughts and observatiosn on the subject. Hope they help some.