President Carter, under pressure from fuel-hungry truckers, is considering suspending a federal rule they can use an adminstration official yesterday.

"The decision has not been made," Bill Johnston, a member of the White House domestic policy staff, told the South Carolina congressional delegation. But he added that he expects the move very soon.

"Rule 9," due to expire July 31, guarantees priority allocation of diesel fuel to formers and truckers hauling agricultural products.

Meanwhole yesterday, a snipper's bullet killed a truck driver on a highway outside Tuscumbia, Ala., in a mounting wave of violence that threatened to escalate into a nationwide walkout of independent truck drivers at midnight.

William Hill, chairman of the Independent Truckers Association, said here is was too late to call off the shutdown because to many truckers' groups were involved.

The walkout was expected to halt the movement of large quantities of produce and gasoline supplies.

Presidential press secretary Jody Powell told reporters earlier yesterday that Carter was considering moves to increase fuel supplies to truckers and would announce a decision before he leaves Saturday on an Asian trip.

Powell indicated that the president was not likely to support the truckers' demands for a higher speed limit and standard weight and load regulations, which are left up to the states.

Powell talked to reporters after Bennett C. Whitlock Jr., head of the American Trucking Associations, urged the president to direct the FBI to investigate violence against truckers who have ignored calls by organizations of independent operators for a work stoppage.

Powell reminded reporters the Justice Department had sent letters to governors offering federal law enforcement assistance if they requested it and to U.S. attorneys telling them to be alert for violations of federal law.

Justice Department spokesman Bob Stevenson said a number of federal laws may apply to the highway violence, including laws protecting the flow of interstate commerce and protecting civil rights.

"We have more reports every day of violence," Whitlock said. "Some carriers tell us they are sending out 50 or 75 percent of their trucks due to the threat of violence or the fear of violence."

Most of the nation's 100,000 independent owner-operators carry freight under contract with ATA members, who act as brokers between shippers and the independent operators. The ATA said it speaks for 16,874 firms which operate 600,000 trucks.

Trucker state troopers said Robert C. Tate of Birmingham bled to death after he was shot in the leg while driving near Tuscumbia an his rig rolled into a cotton field.

Two drivers were injured by gunshots and rocks in Tennessee; three trucks were shot up in Illinois, two in Utah and Kentucky, and one in Pennsylvania.

"Shots are being fired all over the country," said Mac Vernon, a spokesman for the Independent Truckers Association. "Tires are being slashed. Windows are being broken. There's fights. But we are doing all we can to stop the violent aspect. We are telling our people it's not worth getting killed for."

Alabama Gov. Fob James posted a $10,000 reward in yesterday's slaying. James posted a similar reward last week in the case of Georgia truck driver's wife who was shot near Tuscaloosa while riding in her husband's rig.

On orders from Minnesota Gov. Al Quie, who declared a state of emergency, National Guard troops and state police stood guard at gasoline terminals in 15 cities as gasoline tank truck drivers resumed deliveries to service stations.

An official of the Illinois Agriculture Department said the shutdown has had a major impact on movement of livestock, causing the instability in prices.

Late yesterday, the nation's meat packers asked President Carter to provide armed guards on truck caravans.

John Mohay, president of the National Independent Meat Packers Association, sent the president a telegram reporting "drastic curtailment" of beef slaughter because of the violence.

"This immediate relief is vital to avoid further violence and to protect the interests of consumers, farmers, the food industry and independent owner-operators," Mohay said. CAPTION: Picture, A Minnesota National Guard unit jogs outside a refinery in Pine Bend yesterday after being called up to prevent independent truckers from slowing gas shipments. AP