Your DK is right, you could stand to gem a little more stam. You only need 102.4% CTC (block+parry+dodge chance+5) fully buffed. Once you have that much your pretty much left only with Stam as providing any tangible survivability benefit, you're already above that figure. However even with your healthpool an impale shouldn't be a problem. It hits for 400K its been nerfed 20% to 360k, you should be in def stance for another 10% so 324k and you should have demo shout up for another 10% so 291.6k if you use shield wall you knock 40% off it taking it down to a survivable 175k hit.Being as thats 6k less than your unbuffed health pool it shouldn't kill you. Shield wall is a 2 min cd, so should be up for Every platform. As long as your making sure you're in def stance, have demo shout and shield wall up at the right time you should survive, assuming you healers are making sure you're topped off before hand.
There;s quite a few things i'd change with both yours and the DKs specs, but ultimately both will work, they're not the best they could be, but if you're having problems its not going to be down purely to the spec. I susspect its probably down to inattentive healing, poor CD managment, and a lack of tank debuff uptime.