Was following your thread Pam - and Glad you made your final decision - it is really hard trying to wade through all the different opinions.
I think these type of threads should start out with style of driving first
To buy a vehicle to run up mountains at 70mph possibly once or twice a year seems a bit excentric.
When comparing safety - how safe is it to run at 70mph vs say 55mph on the flats. How safe is it to "worry" about the other guy because they may be irritated?? I would be more inclined to worry about the other guy on a defensive driving approach.
While wheel base gives you better stability - these people who drive so fast while towing NEED it.
We have a 1999 GMC Yukon - and have towed both our Globetrotter and the 26' Overlander - with NO issues. We have the WD and Sway system and both setups are nice and stable. We drive a bit slower than our friends to the south and our roads are much smaller. But for heavens sake - you have plenty of LANES down there not to have to worry about "the other guy". And Mountains in most cases I would think there are two lanes on either side - faster traffic can always pass. If not - so it takes a couple of extra "minutes" (not hours) to get up the "hill".
I'd be more worried about coming down the otherside - and making sure you got real fresh brakes on that Yukon/Tahoe - and on your trailer and making sure your brake controller is up to snuf and that you know how to use it safely if you need extra braking power in an emergency. These are all the little things that people forget When they are so hung up on getting the Big TV that can handle anything
Just my two cents - now that it is worth a bit more these days
Happy and safe travels Pam