Xinjiang is an autonomous region of China. It is the biggest province of China but not very densely populated. Most of the province is desert. There is one major motorway going all the way from the border of Kyrgyzstan via Kashgar, Urumqi and Turpan to Gansu province.
Xinjiang is inhabited by the Uyghur people and their language, written in Arabic script has official status here. That means, unlike in the rest of China, motorway signs are not in Chinese & English but in Chinese & Uyghur. It will help a lot to learn at least the major city names in either of them.
Hitchhiking generally works quite well. There are big distances between cities, which means there is little traffic, but also that many cars go very far. It's very possible to get lifts of 300 - 1000 km.
While hitchhiking with Chinese people works just as well as anywhere else in China, hitching with Uyghur people is more similar to Central Asia: They will be very likely to stop, but they will also be very likely to expect some money from you. While many will just stop, ask "How much?" and drive off if you say "nothing", some of them will just pick you up, say nothing, be friendly all the time and at the destination suddenly ask for money, often ridiculously high amounts. That said, it's very important to make it clear that you don't want to pay, before entering a car.
Uyghur people do not understand Chinese language and use Arabic writing system. z-z-z88 (talk) found this hitchhiking plate on some Russian forum and it worked well:
I am traveling by hitchhiking. I have small amount of money and spend it on necessary things only, for example on food. 我搭顺车旅行。我没有多钱，所以这笔钱用最需要的东西，比如食品。 مەن ماشىندا ئوڭوشلوق ساياھەت قىلدىم. كۆپ پۇلۇم يوق، پۇلنى ئەڭ زۆرۈر بولغان نەرسىلەرگە ئىشلىتەي، مەسىلەن يېمەكلىك.
It's quite common for truck drivers to take people between Irkeshtam border crossing and Kashgar for money, which might make it harder to get a lift for free. Taking a truck will still be much cheaper than going by taxi or bus. It's very easy to get a lift for 50 ¥ or less, while a taxi will cost more than 500 ¥. As of 2011, the road is in a very bad state, some parts under construction, which is why the trip will take 6 hours in a car.