It started with a short letter to a local newspaper here four months ago, in which a woman from Ketchum, Idaho, warned Jackson residents that the Federal Government intended to build a nuclear incinerator 90 miles upwind from Jackson and Yellowstone National Park. At the time, almost no one here knew what she was talking about. Now, almost no one here does not, and the shift has led to the latest battle between the Department of Energy and a Western community determined to stop construction of an incinerator that some scientists believe will spew radioactive and toxic materials into the air.