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sdbutler80: Hickory is a troll
Here we go. Another sock puppet.
GawainBS: I like discussing game mechanics.
dtgreene: Same here.

Anyway, back to the main topic:

One nice thing about the Icewind Dale series is that you can change your party mid-way through; if one of your characters isn't pulling her weight, just delete her and create a new character via character arbitration.

Unfortunately, IWD2's XP system is horrendously designed; actually adding a new character midgame will result in the rest of your party getting more XP than they would otherwise. For this reason, if you do decide to create a new character later in the game, I seriously recommend using the cheat console to raise the new character's XP and level to an amount that is comparable to the rest of your party. Yes, it is necessary to cheat in order to avoid becoming more powerful than intended.

Incidentally, if you simply avoid leveling up when you have enough XP, you can continue to get more XP from enemies, possibly getting enough to break the game. I have seen this strategy referred to as "level squatting", and since you mention that you don't want your party to be "too munchkinny", you might want to go out of your way to avoid doing this.

Note that these issues don't apply to IWD1; in that game, the XP system behaves sensibly, there's no advantage to delaying level-ups, and adding a new level 1 character will not risk breaking the game. (If you add the new character no later than level 9, the character will be at most 1 level behind after the rest of the party has gained 1 level, which is reasonable, and the rest of your party will not be leveling faster than normal.)
We can replace characters midway through the game? I spent a couple minutes looking for a way but didn't see anything obvious. How does one do that?

Load your saved game and press "c" or select bottom-left option on in-game menu (below your character portraits). There you can delete and add new characters.