The United Food and Commercial Workers has extended the contract deadline covering workers at Ralphs, Vons and Albertsons for two weeks as the union and the corporations continue discussions. The contract was due to expire Monday night. Among the proposals on the table from the UFCW is a plan to eliminate the current two-tier wage system agreed upon during the last round of negotiations three years ago. Under that plan, new hires are paid less than longtime employees and have a longer waiting period before receiving benefits. The extension, requested by a federal mediator working with the two sides, may be extended again when the two-week period expires March 19.