Fastest leveling I'm pretty sure would be to leave the pally behind and have your 80 DK run the shammies through 5-mans with their rested XP.

That won't get you the cash and uniform gear upgrades of questing, so you may want to do a mix. With a combination, you can just skip all collection quests, even those that unlock quest chains. You can just buy the shammies crafted gear at 80, which is equivalent or superior to all the quest gear.