Melissa,
Stats don't count in altitude sickness. I have spent my life skiing, mountain climbing and hiking at high altitude. Anyone who is a climber can tell you that altitude sickness is unpredictable and I personally don't think having an auto immune disease makes any difference. There are plenty of people with lupus and other AI diseases who live at altitude.
Adjustment to altitude takes time for your blood to produce more RBCs to carry more oxygen and to increase lung capacity. There is an initial adjustment in about 3 days for most sea level dwellers but a more permanent one takes months. The sherpas who help climbers in the Himalayas have adjusted over centuries and many generations and are the only people who can live and work comfortably at such high altitudes. You are going to a high altitude but rather low by comparison.
I have been at 10000+ 3 times since getting MCTD and had no problems related to the disease and plan to go climbing this summer in the Sierras. The shortness of breathe I experienced is no different from when I was healthy and in excellent shape. Maybe you would not want to climb Everest or K2 but 9000' should be OK.
As for taking meds....try to take aspirin. It is the old climbers antidote for altitude and works if you start it a day or two before going. Also stay hydrated...lots of water. Sometimes nothing helps. The worst symptoms you should encounter are sleepless nights and shortness of breathe if climbing stairs, etc. Severe altitude symptoms are rare at 9000'.
Go and enjoy the wedding. My youngest son got married at 10,500 in norther Arizona 8 years ago and it was spectacular watching the sun set over the Grand Canyon.
By the way, Vail/Beaver Creek are about 8000" and Purgatory is about 8800' at their respective bases.
My comments are based on real life experience over 50 years and not on a google search. Enjoy and don't worry so much.
Bill