XTs are common, They are all over the place.
Two people that you might contact who have experience with them is HayDuke, and AnnieGS.
HayDuke's wife Bonnie Abbzug had one for years. They are based in Salida Co. I know that she had that bike in some really high country. They are great people. If they decide to help you. You will be in good hands.
Other things that you might want to consider, include protection, and suspension.
Protection; can be anything from a bash plate for rocks to brush guards for your grips. Shark fins for brake disks are useful too. Lot of rocks in the Rockies. Rocks can be a deal breaker for your ride. Case your bike and it's over.
Next is suspension and overall bike balance; This is a hard one, you will be riding everything from rocks to sand. A good suspension is setup to roll over babyheads and not wallow in the sand. Rolling over babyheads requires a loose valving and lower oil levels in the forks. While loading and sand (which is a form of loading) requires higher spring rates. If you bike wheel chatters on gravel ad preload. Get a good setup guy and see what he recomends. PM me if you want a few names.
Next up is bike balance. I know a little about sag on more aggressive bikes such as KTMs. Not so much on XTs. But be sure that you have your bike balanced for you. If the front sits to low then you it will over stear. If the rear sits to low then it will understear and the front will kickout on rocks.
An out of balance bike will make you ride a living hell. Remember you are climbing and decending. These are two conditions that you might not be ready for.
I have never ridden over Cumberland pass and not had snow, rain or hail. Normally I get all three. Think about where you will be. Sometimes you will be 2 hours out from any heat source. There is a reason that they call anything west of the front range Coldado. Dress in layers. and stay warm.
Lastly ride your skill level. You will be riding with guys who don't know shit. and guys who have been riding for years. Stay away from the guys who don't know shit. And ride at your own rate of speed, there will always be people riding faster and slower. A lot of people have been hauled back to town in the back of a Ambulance. That includes me. Some say that it's only fun if you finish. I can buy into that. More people crash and burn on Dual sport rides than another type of ride.
Lastly bring your camera. And don't be afraid to stop and enjoy where you are. I love back country riding. And Colorado is an amazing place to back country ride.