There is no necessity to revert to a feudal economy or society. The increase in agricultural productivity brought about by the mechanical reaper makes serfdom unnecessary. Modern mechanical reapers were developed in the early nineteenth century, were horse-drawn and can be built and maintained by blacksmiths. A large part of the society can be freed from agricultural labor compared to a feudal economy.
Other than gunpowder, I don't see how the Change would make the US any less capacle than it was at the beginning of the 19th century. Add in developments in chemistry, mechanics, and biology that weren't effected and you have a society that may just be agrarian by simple majority. Remember, the US was very connected before the advent of railroads: toll roads, canals, coastal shipping, rivers, etc. Big cities could still be fed from somewhat far away places (Ohio to the East coast, for example).