Maybe with the december patch so i can increase my xp gain but right now a new level takes a lot of time. But you are right.Level 84 might be low. I'm level 81 and on Act I. Granted the experience gained is a lot quicker with /players 8, but the cap is 99 and you're near the end of the game on the highest difficulty. Might not hurt to get it higher.
And wow, I had no idea you could play a sorc with no points put into energy and make it that far. And I can't find any good items for the Barbarian either. Even if something has high AR, it's for a dexterity or mana based class instead of strength.
You still get 2 points to mana for each level and together with a +150-200 to mana from charms and the meditation aura from my merc this is really doable. Energy shield is also incredibly useful to not die to fast.
The best would be to play with each class alternately and share the gear but who would do that?