Well, you can multiclass the barbarian with a fighter and that will allow him to wear heavy armor.
You could do the same with the Ranger, but that's iffier. If you're playing a high DEX ranger, you may be better off not trying to go for the heavy armor. Also, if your ranger wears anything heavier than light armor, the virtual dual wielding goes away.
All in all, the only characters that "need" to wear heavy armor are your tanks. Hi DEX characters can make out just fine with really good light armor and have just as good an AC as a low DEX, heavy armor wearing tank.
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