Not able to need on a epic in a random.

What you've described, Dranette, is the intended functionality of the revamped "Need Before Greed" system present within cross-realm dungeon groups.

You can read a bit more here: http://forums.worldofwarcraft.com/thread.html?topicId=21627535173&sid=1&pageNo=1#5

Q u o t e:
Need Before Greed

Need Before Greed will be the default and unalterable looting system for all groups formed through the Dungeon Finder (excluding premade groups of five). This system has been revamped and will now restrict “Need” rolls based on several factors:

• The player’s class must be able to equip the item, either at the time of the roll or at a later level. For example: A level 38 shaman would able to roll “Need” on a level 42 mail item; however, he or she would not be able to roll “Need” on a plate item.
  
• The armor proficiency of the player’s class must not be higher than the item being rolled on. If it is, the player will unable to roll “Need.” For example: A level 75 warrior would unable to roll "Need" on certain mail items. (This restriction will only come into play for items with an iLevel above 70.)
  
• If a player’s class is pure melee, he or she will be unable to roll “Need” on spell power items. Pure melee classes include rogues and warriors. Druids, paladins, death knights, and shaman may still roll “Need” on spell power items, provided that the above two conditions are met.

All players may roll “Greed” or “Disenchant” on items which drop within the instance, however, regardless of class, level, or armor proficiency.

As neither priests nor mages cannot equip shields, both classes will be unable to roll "Need" for them. "Greed" and "Disenchant" rolls are still available, however.

My sincere apologies for any confusion this may have caused. Please know that with patch 3.3.3, "[i]f a random party is created via the Dungeon Finder and a player cannot roll Need on an item, a reason will now be provided." (Source: http://forums.worldofwarcraft.com/thread.html?topicId=23767643755&sid=1&pageNo=1#3) :)

Q u o t e:
But Syndri, I have an honest question: What if no one in the groups actually needs the item, and everyone wants to sell it on the auction house for gold. Isn't it a bit unfair to those classes that can't need on it because of this new system?

The alternative was allowing everyone to roll "Need," as with the previous system. This, in turn, can create situations where players who simply want to sell an item are given an equal roll as those who actually want to equip it.

With that said, though, if you have any recommendations for how we may fairly accommodate for both parties, those who wish to sell and those who wish to equip, please do let us know via the Suggestions Forum. The revamped "Need Before Greed" system is still new, and we're always looking for ways to improve it. :)