I had 3 Highlander Limiteds since 2001. Tried an Explorer before and found the ride to harsh for my liking. Was reading the newspaper at work in November 2010 and saw an article on the new Explorer saying that it no longer used a ladder (truck) frame and was now unibody. What really got my attention was a full colour photo of the dash. I more or less said to myself, I've got to have that. As it turned out the fellow that bought my '06 Highlander at lease end was standing there and said he would take my '09 lease over. It had 2 1/2 years to go.
The Highlander in my opinion had a little more 'get up and go' than the Explorer although both have the 3.5 V6. I also initially felt that the Explorer rode a bit stiffer but now that I've had it a year, I don't really notice it. Fuel mileage is about the same. The Explorer is wider (3.5"), longer (9") and taller. The Highlander didn't have any of the 'growing pains' we are currentl experiencing with the Explorer. It really was a great vehicle. The main difference is that unlike the AWD Explorer, the Highlander was 4WD. All 4 wheels were powered all the time, 50% front and 50% rear. It also came with a full sized spare and a 3rd row, which I never used. Towing capacity is 5000lbs and 8.1" ground clearance. Cargo room behind 1st row is 95.4 cu. ft compared to 80.7 for the Ex.
Hope that answers some of your questions.
Peter