I really do not think the HiHy is justified in your situation.
It is not often that I steer people away from Hybrids because I like them so much... but really, to save gas... well... you're out of luck with what you need. Now, if you could get by with the HiHy for those summertime long family vacations ( the HiHy can tow... 3500 pounds? ) then you will be much happier over the Yukon.

Of worthy note... I don't think the Highlanders are that big.
They are more of a station wagon than SUV. yes?

What about a mini-van with AWD? I think those are best for large families... with soon to be pre-teen and teen age kids. How long do you traditionally keep vehicles? I think you may out-grow a Highlander before you wear out a Highlander. I think the Ford Escape is a much better value for the money also... but is even smaller. Good Luck,