I've been playing for about 4-5 hours with a friend and he's levelled a full level above me.
Is there either any way to ensure we level equally or any way I can catch up with him without taking our quests out of sync, like farming mobs not related to quests or any mods that could help?
+x% exp
gear to slowly even the odds if you find yourself falling behind repeatedly. The adventuring scroll can help with this.