I'm still early in the game, but it seems there are some built-in methods for catching up characters who have been "left behind" in levels. You can have them take pot shots at enemies out of their league, because they'll still get experience for the hit even if it does little damage. If a character knows magic, you can have him spam that (even healing) to gain experience without direct battle. Do you get experience for using items? I haven't tried it yet, but it seems possible given how the game rewards everything else. If all else fails, you can wait until new equipment is available for the character, and purchase that, which will artificially boost his stats, hopefully to the point that he can be somewhat useful in combat and start to catch up.
I do think the leveling up could get lopsided, due to the fact that killing an enemy provides far more experience than doing damage to it. But as gtownclown explained, this system can also be leveraged to boost characters who have fallen behind.