Is country/peasant worker vs city worker an ethnic divide? Sounds more like a class divide. (ironic in a post-maoist country)
The urbanisation figures in china are highly suspect. Given one child policy is primarily policed in the city and intentionally lax in the countryside. This coupled with the fact their census seems (unconfirmed) do NOT count unofficial residence (i.e second/third/etc children). It makes it very hard to get a real picture of China's urbanisation stats.
Is country/peasant worker vs city worker an ethnic divide?
The distinction you're looking for is "Han Chinese" versus "everyone else." China's outlying rural provinces have significant populations -- often pluralities or majorities -- from ethnic groups which are not Han Chinese, which is the dominant economic group in China and which is the portion of China largely winning from economic growth. For example, there are several large, predominantly Muslim ethnic groups in the border provinces, by the *stans and what have you. Plus Tibetans, etc, etc.
It is possible that this ethnic conflict eventually gets resolved like the Yamato vs. Everyone Else conflict in Japan: the nation of Japan became coextensive with Yamato Japanese and everyone basically pretends that Japan is monoethnic. (This would have been Serious News To Us for most of Janaese history.) It is also possible that China goes along a more Russian or Balkan path.
It is also possible, I suppose, that China resolves its ethnic issues by some combination of ethnic cleansing and genocide, which would not be new.
On a macro-scale the non-Han are relatively small, though, especially compared to some of the other cases of multi-ethnic countries. Non-Han-Chinese make up only 8% of the PRC population, whereas non-Russians make up about 20% of Russia's current population, and made up about 40% of the USSR's.
It is true that they're much more prominent in the outlying areas; e.g. 60% of Xinjiang is non-Han. Those areas have relatively few people compared to the more populous parts of China, though; Shanghai alone has more people than all of Xinjiang. China's current strategy seems to be to take advantage of that numerical imbalance to Han-ify the outlying areas, since the smallish (in absolute numbers) non-Han population can be swamped by moving only a few percent of the people from the central cities out there.
It's easy to throw out numbers like 8% until you realize that it represents over 100 million people. Per capita that may be a small number, but in the general scheme of things it means a lot of disgruntled people. I never really thought about it until I went over there and had the chance to interact with various groups, and it changed a lot of my perspective on how China functions on the inside.
I grew up in Xinjiang, arguably the most ethnically divided region in China, so I know where you're coming from. China's approach to taming ethnic minorities is straight forward: various forms of oppression. Besides racism, the other major reason is that minorities in China, unlike those in the US for example, do not want to be Chinese. There's no civil activism because neither side want true equality (both want to be separate).
Back to the original point: the Han peasant class is far greater than any other demographic. That demographic has been responsible for the dynastic cycles for thousands of years. It's still the case today, and the Chinese government is well aware of that. At the same time, the government has to keep stoking the economic fire, promote innovation, etc.
China's ethnic groups are small, weak and concentrated in remote areas. They really are probably not a political factor. I really can't see this ever being the dividing line of a civil war. They might demand autonomy or independence but that's not really a civil war.
Any serious civil strife that could effect the political direction of the country is most likely to be of the revolutionary kind. The most likely (though still not that likely) scenario I can think of is post economic downturn combined with political scandal. It would probably be a nebulous combination/coalition of demands/factions. Pick a basket of anti regime, pro peasant, anti corruption, pro democracy, human rights. Maybe Christianity or religion in general plays a role.
I think you need to draw a line though between civil strife and revolutionary agitation. I think the urban/rural divide will most likely manifest in the latter.
The urbanisation figures in china are highly suspect. Given one child policy is primarily policed in the city and intentionally lax in the countryside. This coupled with the fact their census seems (unconfirmed) do NOT count unofficial residence (i.e second/third/etc children). It makes it very hard to get a real picture of China's urbanisation stats.