go for it, they (and you) will love the trip. Heck my old 2004.5 is now our son's and it has 160K miles on it and neither he nor I would hesitate for a second about a long trip. Opps, he takes it on long trips quite often....
The 8.1 is a workhorse, and the mountains going through from the east to glacier, yellowstone, etc. aren't that high. the continental divide passes in montana are in the 5-6K range, with decent 6% grades. there are a few that are higher, and going from billings to yellowstone, you probably don't want to take that pass into yellowstone, pretty windy and around 10K elevation. But you can easily find routes you want that won't be any struggle.
