What is the most fuel effecient vehicle that is also 4wd or AWD with
the most interior cargo room and can hold 8 people (or 2 adults and 4
large carseats comfortably)?

Consumer Reports has a publicly available (no membership required)
answer to your question at
http://www.consumerreports.org/cro/cars/new-cars/best-in-class-cargo-and-mpg-205/overview/index.htm.
It's an article called "Best in cargo capacity & fuel economy".

I've been looking also, and have just about decided on the Honda Pilot
- although it doesn't have very good efficiency ;-).