This is a one tank fight first of all. The DK should go DPS and the Paladin (shield wearing tank) would typically be assigned to be the only tank since they will have considerably higher survivability against the worms that spawn out of the abominations. The abominations and the worms are the only things that require an actual tank. The casters can be handled by DPS and the zombies are kited by ranged.
Is your balance druid only resto for this particular encounter? If so, they should not be using a traditional healing spec but rather a spec designed for higher raw throughput. If the balance druid is in equivalent gear to the priest they should actually be able to outheal them even in balance gear if speced correctly in their off spec. Also, if you're actually approaching soft enrage where adds spawn faster and faster, this typically occurs around 7 minute mark and by that point your healers should be in the 40s for their stack of Emerald Vigor. If they are only in the twenties they can improve on that. With only two going you should be averaging at a bare minimum 5 stacks per portal phase. If you are falling apart much sooner than that then it is because your DPS isn't handling adds effectively enough. So either your healers aren't stacking the buff as fast as they should be or your DPS isn't able to keep up with the adds as long as they should.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.