Now, with U.S. unemployment at a 25-year high, they are also fiercely coveted. American workers -- who for years have largely avoided fruit -picking, office-cleaning and meat-processing shifts -- are increasingly vying for these jobs with immigrants, creating flashpoints in places like Shelbyville, Tennesse. The rising friction has been on display at the employment center here. When Tyson put out a call for applications, in February, shoving and cursing broke out between locals and immigrants jockeying for position at the head of the line. With too few jobs to go around, outraged locals demanded that the work go to residents . . .