I'm sure you are aware all TV's loose some powertrain performance towing at higher elevations, even under ideal TV/TT towing conditions. It should be noted that in some TV owners manuals they say to reduce the TV's combined weight rating (GCWR) by 2% for every 1000 feet.
Everyone's TV towing expectations can very under different towing conditions.., but a good place to start is knowing where your TV/TT combination falls with your specific TV weight limits. I guess one way to look at it is if you are towing at/near your TV's limits then I would expect that reduced TV performance will be even more noticeable.
CAT Scale How To: http://www.jaycoowners.com/showthrea...igh-Your-tt-tv
Since your trailer is 5 years old, having the wheel bearings and brakes inspected/adjusted prior to a mountain trip would be wise..., especially if it hasn't been done in the last two years.
Also, transmission fluid tends to break-down faster on vehicles that are used for towing, and this viscosity break-down is accelerated when towing frequently in the mountains. If your 2007 Explorer has been used as a tow vehicle for a couple of years, then I would recommend having the transmission flushed and new fluid/filter installed. This will insure that the fluid is providing optimum protection and fluid temperature control. The majority of the miles I put on my TV are for towing, so I have my transmission flushed around every 30K miles..., plenty of hill and mountain towing as well.
I agree about taking your time towing through the mountains, gives you time to enjoy the views