How do I unlock Heroic dungeons for previous expansions in the LFD Tool? I currently own Cataclysm, but I'd like to be able to do the Heroic dungeons in WotLK. I have a level 84 character. Am I too high in level to run dungeons from previous expansion? I can only find dungeons for Cataclysm at normal difficulty in LFD.
You're only able to queue for heroic dungeons at the level that they originally required using the LFD tool, with the exception of TBC heroics.
TBC Heroics are available at level 70-72, while WotLK heroics are only available to level 80 players. See this page for a full reference.
You can still go there and enter the dungeon by yourself, without the help of the Dungeon Finder. There's no upper level restriction for any dungeon as far as I know.