As far as the gas milege thats part of the reason I'm waiting for the 8 speed trans. What I mean by more soild feel is a more solid feeling body structure less flex in the body. This could be because the Explorer is based on a Taurus and the Durango is based on the Mercedes M class. They are both nice in different ways but I just think the Durango has the edge. I think the quality just seems better on the Durango.
No offense, but it doesn't sound like you did much of a test drive on the Explorer. Also, what models were you comparing? The interior quality of the 302 package Explorer Limited is definitely much better than the Durango Citadel.
That said, based on what you're saying you want out of a vehicle, you'd probably be better off looking at the Grand Cherokee. It's a little more suited for offroad than the Explorer or Durango. The Durango didn't impress me when I went in to look at it. Mind you, I was in the market for a fully-loaded Citadel or Limited an ended up going with the Limited.
As for a frame being "based on" another vehicle, that pretty much means they take a pre-existing design file and rework it on a computer before fabrication. It's just a basic blueprint for where to start (so they don't need to go from the ground up and retweak a million times over during design/safety testing). The Grand Cherokee and Durango being based on a Mercedes frame design honestly means nothing regarding materials or craftsmanship.
Again, though, I'd really recommend looking at the Jeep. I definitely preferred it over the Durango and the Overland Summit had some great interior materials and the adjustable ground clearance was sweet. I ultimately chose the Explorer over it, as 99% of my driving is on the road. The Explorer was way smoother, quieter, more responsive, and had better gas mileage. If 20% of my driving was offroad, I would have definitely gone with the Jeep.