You can't use fuel prices as the sole measure as to why people start/stop buying certain types of cars. If that was the case, why didn't the sales of full size, body on frame cars rise in the 90's when fuel prices stabilized? Oh, right, there weren't that many models left rolling off the assembly line.
2005-2008 weren't just years for rising fuel prices, but arguably the first real successful years for crossover SUV's and hybrids. The federal government was supporting their purchases with tax incentives, and manufacturer's were offering their own discounts to spur purchases. Add fuel to the fire with cash for clunkers, and you've created an environment that spurs the manufacture and sales of fuel efficient vehicles, and shifts demand away from vehicles without incentives and lower fuel efficiency.
Times changed, and people wanted the latest trendy vehicles that not only saved them cash at the pump, but were literally given cash (via the giv. and mfr. incentives) to buy.
On the flip side, I think we'll see a slight reversal of the trend with the new Tahoe, Suburban and Escalade, they've been recently redesigned and have a strong marketing campaign.
Ford hasn't really invested anything into the Expedition since it's complete redesign in 2003, and it's facelift/interior enhancement in 2007. As a matter of fact, it still had the 5.4 until this past year's 2014 model, which saw the Ecoboost and 5.0 V8 introduced along with yet another minor facelift.
