A few things:
1. Use Shockwave
2. Use Shockwave
3. I'd shuffle some talent points around.
a. If you already went to impale, go all the way to Deep Wounds. Drop 3 points from cruelty and put it into deep wounds.
b. Max incite and improved thunderclap. You can drop 3 points from shield spec and drop puncture altogether. The block rating from shield spec isn't useful in ulduar and the threat you get from puncture will be more than made up for from improved thunderclap and max incite. Plus improved thunderclap = less damage taken.
All of those talent buffs will greatly improve your thunderclap damage. Use cleave and tab around and throw a good shield slam on any mob you seem to be losing.