Need advice on best team setup

I am a newb at the higher levels since I am a constant restarter, but I have a team of 3 Pallys and 2 Shamans approaching level 55. I have them all melee (originally had just the tank melee, but with the click to move, I have the shamans as enhancement and 2 of the Pallys as ret).

So, I am debating getting a DK for this group. Is is worth it? I like the extra aura and buff from my pally (not willing to drop one of the shamans because of utility). If I do drop a Pally, do I drop my main and have the DK tank with two retadins? If I make the DK a DPSer, I was thinking this build...

http://www.wowarmory.com/talent-calc...33001003103150

Which is a 27/26/18 build that takes as much advantage of the DK's auto attack as possible. Basically, would it be easier to have the DK tank and just spam instant casts on the 2 Pallys, or go with this build and have the DK just work his auto attack more with my pally as tank? Or should I just stick to 3 pallys?

Probably not what you want to hear but I think you need to try the DK out and see if you prefer it to the paladin.

All the tanking classes in the game are very capable and so it is more an issue of personal preference.

I love tanking on my Feral druid for my higher level multibox team (if I ever get it going), however I've really, really enjoyed having a paladin tanking
for my shaman. Aggro is never an issue and the team can go all out instantly. It's even got me thinking about respeccing my bear tank as cat and
swapping the paladin to Prot (but I'd need a new set of gear on the pally).