I hope this doesn't fuel the fire because I'm not as knowledgeable as the rest but have you considered the tuning options which were presented? There was one that seemed to keep you in 4 cyl mode longer. As well, a good custom tuner can probably improve the efficiency of both the 4cyl and 8cyl modes. I got a huge benefit in MPG from PCM tuning alone. It's at least a good place to start.
I know you're trying to go that extra mile and some of it really comes down to you being my kind of guy who likes to tinker and make things better but there are a lot of variables missing when things like 4WD hubs and extra CV axles are being thrown around...what do they weigh, how much power to spin them, will used $100 hubs and bearings really hold up as long as what you have, etc.
Good luck, whatever way you go.
I've just been down the road too many times where I'm trying to out-do what is really sensible and I figured one more detractor post would probably make you more determined to do it.
