The government/CITS response to the query about bringing your own vehicle to China and driving it around has been standard for a decade to my personal knowledge, and probably longer. You are quite simply not permitted just to show up in China with your own vehicle and drive around. And there's no point in thinking, as hinted by others, that you'll be able to bribe your way around it. You won't. The only way to get to drive around China in your own vehicle more or less at random is to have a residence permit for China, a Chinese driving licence, and a vehicle provided by your employer, purchased by yourself in China, or rented. Even with rental venturing far away from the point of rental can be tricky.
It might also be added that only the insane or those with long experience of Chinese traffic tackle driving in China, where there is simply no manoeuvre so absurd or suicidal that it isn't performed with considerable frequency. The only strategy is to assume that every other vehicle is being driven by killer zombies. Driving is best left to natives.
If you do arrive in China with your own vehicle you must first have everything mapped out and booked with CITS. The requirement to have an accompanying vehicle, driver, and guide are absolutely standard. If you are travelling in your own car you can get away with just the guide, who, if your journey is an extended one across China, will indeed have a briefcase full of permits and paperwork.
CITS will map out the entire route, and require you to follow it directly, booking all the hotels for you and throwing in day tours at various cities. You can ditch the day tours, but almost certainly not the hotel bookings, which will of course be at the 'best' hotels and at the most inflated prices. You ought to be able to negotiate down the quality of the hotels, but you will certainly be paying several times more than you need to for them.
I've come across two examples of this procedure in progress in Xinjiang itself. One was a convey of three Land-Rovers with British soldiers in them making their way from Hong Kong to London at the end of a tour of duty, and raising money for charity at the same time. It had taken them 18 months to make the arrangements, and they were accompanied by a CITS guide with a large mound of papers.
The second was a pair of Dutch bikers, one on a motorbike and side car, the other on a regular bike, who had driven from Holland to the Kyrgyz border with China. They dealt by fax and telex with a Kashgar-based agency with the standard inflated ideas, and in the end took the option to have the agency bring a truck to the Torugart Pass border, on which the bikes were loaded. The cab then had to accommodate driver, guide, and the two of them, so they threw the guide out in Kashgar. They also rejected the day tour there, and downgraded the hotel. It was only one over-night stay before heading on to the Khunjerab Pass with Pakistan, and getting back on two wheels at that point. Despite intense negotiations the price was in four figures in US dollars.
In short, pay up and put up with a fixed itinerary, or forget about it. Or do it by pushbike not motorbike, which is permit-free and just a matter of climbing on and pedalling off (as well as being better for the heart, lungs, and environment).
There is no particular added difficulty in including Xinjiang in your plans, however. And the road border crossing to Kazakhstan is not noted for difficulties with anything but the rapacity of the Kazakh border guards eager to find that you are not crossing into Kazakhstan on precisely the day specified in your visa.
Peter N-H
http://www.datasinica.com
http://peternh.blogspot.com