Save your money from now to 80, and go buy the Tempered Titansteel Helm and Treads. Get the rest of the tempered saronite stuff (I think there's another piece or two you're missing), and run SLabs till you get the Adamantite Figurine (I think that's it) from Blackheart (at 80, I could get to him in just a couple of minutes if you're careful with where you walk) for a little more defense. That's basically what I did, and while they were a lot harder than they are now, it was doable with that as a start.

Buy the Wyrmrest tabard, and get the chest you get at revered (or Moorabi I think drops a defense chest I wore for a bit).

Your shaman look a little better than mine did when I started, so you'll probably be fine there. Just bind all 4 shaman to lesser heal the tank for when she takes spike dmg (and she will, DKs seem to be very spiky with taking damage, or at least that's what I've noticed).

Like others have said, you really need more hit points. Some of those bosses have abilities that hit tanks for 10 to 15k iirc.

EDIT: Looks like you have all the saronite pieces after looking again. Get that figurine from Blackheart the Inciter (32 defense) and probably the epic helm and boots (Tempered Titansteel stuff).