NEW YORK (Reuters) - Washington Mutual Inc. (NYSE:WM - News), the largest U.S. savings and loan, said on Wednesday it will refinance up to $2 billion in subprime home loans at below-market rates to help borrowers who might otherwise struggle to keep up with payments. The decision comes amid growing defaults nationwide by homeowners with weaker credit histories as gains in housing prices slow, rates on many mortgages adjust higher, and lending standards tighten.