I suck at PvE - please help my shadow priests

[Owltoid]

Granted, my gear sucks, but I should have no problem running regular UK (yes, the level 70 instance) using my 80 druid and 4 79 shadow priests. The problem is my priests are running out of mana like crazy. I can do the whole instance without wiping, but I have to pretty much drink between every single pull, which kind of defeats one of the goals (to get gold before Cata).

Please help!

My priests (for armory gear):
Thisuglyguy
Thatuglyguy
Whichuglyguy
Otheruglyguy

My "tank":
I think it's the Owltoid where the i is the left slash hash mark (alt-141)

A typical group of trash mobs goes like this:
Run in with the druid, and basically spam swipe the whole time (yea, this needs to be improved, but focusing on the SP right now)
After a couple seconds of aggro, the SP all cast renew on the tank
Cast sequence for the mobs is SW: Pain, Vampiric Touch, Mind Blast, Mind Flay, Mind Flay, Mind Flay (mob is usually dead)
If the shit hits the fan, then I holy nova or shield themselves as needed.

I typically stay out of shadowform, but if it's going to help that much then I can go into it. Typically that one round of renews is enough to keep the tank alive with the passive shadow healing.

My gear is god awful, I mean really bad. I haven't looked at my druid's tanking gear, but I bought it basically just to boost. My priests gear is mainly quest gear (up to level 75 or so) but focused on being holy/disc build (my original plan before the beta changes).

Why am I running out of mana so easily? Is it just a function of gear and I need to put up with it until I upgrade?

The other goal of doing this team is to get practice so I can hit the ground running in the expansion. I'm not looking to gear these toons out, and I have very little gold so AH boosting isn't a big option unless it will help a ton.