Pally / Mage / 2 x Lock / Priest - improve?

[wakasm]

For the most part - instances until 80 have been easy. I try to take the first run on all new instances with this team fairly seriously (CCing, etc) just for practice purposes - but nothing has given me trouble as of yet, but it quickly devolves into mindless runs - and I'm assuming that is because it's pre-cata content.

I chose priest just because I know priests better (my main was a priest) - I have 5 other teams all waiting at 60, but I have yet to roll a DK.

My main complaint is that I feel like I will quickly get frustrated with gearing up 3 caster dps in cloth, so I was trying to figure out how to swap in a 5th class that could maybe utilize leather or mail.

I guess overall I love playing 5 distinct classes or 5 of the same (My next favorite team has been my 5 feral druid team).

I was also considering another priest - shadow?

I guess I just don't know enough about DK's to know if replacing a Lock would be any benefit. I don't mind adding a melee - but I feel like a rogue/hunter might be too much due to the spacing/movement issues.