It's easier to ride a 450 at 11,000ft than anything else. Also, it's good to have a plate on it in CO. 250-450cc bikes are all the same chassis size. I'll suggest a Honda 450x. If mountain passes and such are on your mind. There are a few of us in the club that ride ktm 450exc's 2004-2007 I'll suggest one of those too, but are more difficult to lower.
98 uzj100. Factory rear-ARB front locked 4.88's. ARB Bull, Warn 9000, Irbis rear bumper, Slee sliders and belly pan. OME t-bars w/863 rears, HID, XM