You are not the first one to complain about this. I've heard some stuff about Blizzard changing the XP share when a high level character is in the group, so that the lower level characters receive negligible XP. I'm not sure if its true but I heard a complaint this morning about a friend having the same issues as you (not receiving as much XP as he used to).

Looks like RAF boost leveling has come to an end....sadly