Roll a death knight tank and join him up with your 4 shaman once he leaves the starting area.

Get used to the DK tanking with 4 ele shaman, then when comfortable with that swap a shaman to resto for harder instances.
I've gone for the resto shaman now in BC instances but you can get away with not swapping till northrend.

The reason for the DK suggestion was the fact you can start him at 55. I would create him on your existing account then transfer him to your new account (although you will need wow, bc and wotlk to do so. Close to Zero downtime for your team is well as you wont have to wait too long to level him.