Wrath of the Lich King gear is really bad in terms of iLevel compared to Hyjal questing greens. ICC Heroic gear is on the same level as Hyjal questing greens. I recommend doing the entire zone of Hyjal(or Vashjir, but I hate Vashjir), to get decked out in 278/288 gear. The stamina difference in itself is huge, as well as the spell power/attack power difference you'll see. Being a triple boxer I don't much know what its like doing 5 mans by myself, so I can't really comment on the gear required to tank and heal dungeons at the same time, but if its anything like playing DPS, you're going to want to overgear the instances you do as much as possible, which I know sounds a little ridiculous. You're undergeared(from what it sounds like), and the gear should make immense differences. Not to mention that 80-82 is relatively quick(compared to 82-85).