Energy Drain and Negative Levels
Some spells and a number of undead creatures have the ability to drain away life and energy; this dreadful attack results in “negative levels.” These cause a character to take a number of penalties.
For each negative level a creature has, it takes a cumulative –1 penalty on all ability checks, attack rolls, combat maneuver checks, Combat Maneuver Defense, saving throws, and skill checks. In addition, the creature reduces its current and total hit points by 5 for each negative level it possesses. The creature is also treated as one level lower for the purpose of level-dependent variables (such as spellcasting) for each negative level possessed. Spellcasters do not lose any prepared spells or slots as a result of negative levels. If a creature's negative levels equal or exceed its total Hit Dice, it dies.
A creature with temporary negative levels receives a new saving throw to remove the negative level each day. The DC of this save is the same as the effect that caused the negative levels.
Some abilities and spells (such as raise dead) bestow permanent level drain on a creature. These are treated just like temporary negative levels, but they do not allow a new save each day to remove them. Level drain can be removed through spells like restoration. Permanent negative levels remain after a dead creature is restored to life. A creature whose permanent negative levels equal its Hit Dice cannot be brought back to life through spells like raise dead and resurrection without also receiving a restoration spell, cast the round after it is restored to life.
So, you're right the barbarian definitely retains the ability. And you're also right that the question of "Is it a level dependent variable?" is the crux as to whether a rogue can sneak attack him or not.
It's not clear that there is a correct answer, but I think I would probably say yes. The barbarian counts as 4th level not 5th.
In this situation the correct tactic is to keep moving and not let the enemy get a full attack, especially if it's a twf rogue. He is dependent on positioning and full attacks to have damage that is as good as a barbarians. If you move away and force him to spend a move action to follow he will no longer be flanking and will no longer be able to full attack. You're single attack damage will be greater than his, so eventually you win.