Yes mostly mutton... delicious flavorful lamb or mutton (difference is after a year old - it is mutton) - i am sure mongolian lamb would be good! Tibetan food is not so close to Uyghur cuisine. The Himalayan cuisine of Tibet is influenced greatly by their climate and difficulty to grow much produce... but both cuisines utilize lamb... while Tibetan may use Yak more. (dryer than beef -- not my fav)
Ultimately, the influence of Chinese cuisine and flavors has a whole lot to do with both Uyghur and Tibetan, and their cross culture flavors... perhaps this is the reason why it is so damn good!

the noodles at Jibek Jlou were pretty disappointing...

Thanks for the quick reply! I actually found that restaurant soon after my chow post. Looks really cool and unique! Will def try soon. Can't wait to see how the lagman (Gan ban mien) compares to the ones I had on the streets of shanghai served by my Uyguhr buddies!

Man, I woke up today craving some xinjiang street food. Damn- Chicago has just about every good but this one lol!!