The U.S. Postal Service's governing board, stung by the resignation of its vice chairman, who pleaded guilty last Friday to embezzlement, has ordered a wide-ranging review of how the mail service buys everything from mail sorting machines to delivery vans.

The probe, announced in a statement by board of governors Chairman John R. McKean, will also review the selection earlier this year of Albert V. Casey to be the third postmaster general in less than two years. Casey was recommended by an outside executive recruiter, William A. Spartin, who also once headed the public relations company accused in court papers of funneling money illegally to the board's vice chairman.

McKean said there was no indication that Casey himself was involved in any impropriety. The board fired Spartin's firm just as it had begun a search for a successor to Casey, who is expected to step down later this year after only months on the job.

The vice chairman, Peter E. Voss, a Reagan appointee to the board, pleaded guilty last Friday to three felony counts of defrauding the government in connection with a scheme to steer a contract to a Texas-based company whose public relations firm was paying Voss a kickback.

The unprecedented investigation of internal corruption in the postal service will be headed by Joseph A. Califano Jr., the high-profile former Cabinet member who serves as the board's counsel.

The inquiry comes at a time of turmoil and uncertainty for the postal service, which has seen revolving-door leadership changes at the top, friction between the postmasters and a newly assertive and somewhat more ideological governing board, and intense economic pressures in the highly competitive information-delivery marketplace.

In announcing the investigation, McKean said, "The postal service spends billions of dollars on procurements, and recent experience sadly demonstrates that we are a tempting target for those who are prepared to violate the law in return for self-enrichment."

The inquiry will review all decisions in which Voss played a role.

The scandal largely overshadowed another significant postal development yesterday -- the announcement of Casey's long-awaited reorganization plan that will essentially shift decision-making from what has been called the bloated headquarters and regional staffs into 74 new local divisions.