Mortgage restructuring for strapped homeowners has emerged as a rare growth area in the economy as companies in the field keep hiring.

Four of the largest mortgages servicers -- Bank of America Corp., Citigroup Inc., J.P. Morgan Chase & Co. and Wells Fargo & Co. -- have collectively hired almost 17,000 people this year, mostly to work with financially ailing homeowners. With the number of defaults rising, many are planning to keep adding staff.

"We've...