After skirmishes in Wisconsin, Ohio and Indiana, the next big union battle is brewing at your local post office. The U.S. Postal Service’s two largest unions blasted the financially strapped agency’s proposal to cut as many as 120,000 jobs and pull its workers out of the retirement and health benefits plans covering federal workers for a new benefit systems. The Postal Service, which is facing a second year of losses totaling $8 billion or more, would need congressional approval for its plan and cooperation from the postal unions, which have contracts that ban layoffs unless Congress intervenes. But the American...