Hmm, well if you in isntances, you should have all your guys in Frost Presence. But yeah without Dark Command you will struggle. Once you get it though, you can round robbin it to pass agro on to each of your toons and spread the damage around. When everyone gets low, rune tap the health back up (spec'cing Imp Rune Tap and having the Rune Tap Glyph is a must).

If you're going to have one of your DK's tanking for another group (like me) 540 defense is needed to be uncrittable by raid bosses. But if you are only running heroics, then 535 defense is all thats needed. Either way, at lvl 80 Defense should be the priority stat until capped...then start looking at Stam (you want 25K+ unbuffed for raids) / Strength (for threat) / Dodge / Parry / Expertise (reduces the chance a mob will block/parry your attack) and Hit as well. The good news is that a lot of tanking gear is either crafted / BOE / rep rewards.

Cheers,

S.