The unemployed need not apply.

That is the message being broadcast by many of the nation’s employers, making it even more difficult for 14 million jobless Americans to get back to work.

A recent review of job vacancy postings on popular sites like Monster.com, CareerBuilder and Craigslist revealed hundreds that said employers would consider (or at least “strongly prefer”) only people currently employed or just recently laid off.

Unemployed workers have long suspected that the gaping holes on their r...