In a vote of confidence, the shareholders of the Fruehauf Corp. have reinstated W. E. "Bill" Grace and Robert Rowan, both of whom were convicted in a major tax fraud case.

Receiving 88 percent of the vote, Grace was returned as chairman and Rowan as president and chief executive officer of Fruehauf, the world's largest maker of truck trailers. The changes are effective immediately. About three-fourths of the company's 12.1 million shares were voted.

The action came as no surprise. The company's directors decided last month to back Grace and Rowan who, for the past six months, have been serving community service assignments as a result of their convictions. They had resigned their positions at Fruehauf last November.

The two men were convicted in 1975 of conspiring to defraud the government from 1956 to 1965 of $12 million in taxes owed by Fruehauf. It was a major victory for the government, and marked the first time this kind of tax case had been brought against a publicly-held firm.

The Detroit judge who heard the case at first sentenced both executives to six months in jail. But three years later, after several appeals had been tried unsuccessfully, the judge changed the sentences.

Instead of being sent to jail, Rowan, 57, was allowed to work in a Detroit center that rehabilitates alcoholics and drug addicts, and Grace, 71, was permitted to teach high school kids in Whitney, Texas, near a ranch owned by the foundation that bears his name.