The Defense Department plans to require commercial truck drivers who transport thousands of loads of machine guns, grenades and other munitions each year to carry shotguns and to resist, "insofar as humanly possible," any hijacking attempts on the nation's highways, according to a Pentagon memo.

The regulation, due to take effect June 1, reflects growing Defense Department concern that light automatic rifles, mines and other ready-to-use weapons are vulnerable to terrorist theft while in transit, officials said.

But the demand that civilian tractor-trailer drivers defend shipments with shotguns against "tampering, pilfering, or sabotage . . . or violent disturbances," as the memo from the Military Traffic Management Command urged, worries some truck companies and at least one member of Congress.

While agreeing that shipments may be vulnerable, although there have been no documented hijackings, a trade association director said that arming drivers is not the answer because they are unlikely to risk their lives.

"I don't think it's realistic," said W.J. Burns, managing director of the Munitions Carriers Conference Inc. "All of our members would definitely rather have the police involved."

The new rules sent to freight companies Feb. 12 require training in "evasive driving" techniques. That was dropped in a March 4 amendment, but drivers still will be expected to carry a shotgun and "a minimum of five (5) rounds of appropriate ammunition, readily available for immediate use."

Rep. Cardiss Collins (D-Ill.), chairman of the Government Operations subcommittee on government activities and transportation, said the Department of Defense also appears to be trying to avoid liability for problems during weapons transport.

"If the threat to such shipments is as genuine as the Pentagon apparently believes, the issuance of guns to civilian truck drivers does not begin to address that danger," Collins wrote to Defense Secretary Caspar W. Weinberger recently. "Secondly, equipping drivers with weapons and a mandate to protect their cargoes invites the possibility of unnecessary violence with significant risk to motorists and bystanders."

Unlike nuclear weapons, which the Department of Energy transports on specially armored trucks with government drivers, conventional arms and explosives are carried by general freight-hauling companies under contract to the military. There were about 45,000 such shipments in 1984, double the 1980 total, according to the National Transportation Safety Board (NTSB).

Col. George R. Kaine, spokesman for the Military Traffic Management Command in Northern Virginia, said that armed drivers have been required for several years for missile and rocket shipments, and the rule is now being extended to other light weapons. But there are only a few hundred missile shipments each year, compared with more than 4,000 other shipments that now would be affected, Burns said.

The new rule stems from "a collective feeling on the part of the services that increased security was required," Kaine said. Asked why, he replied, "I guess you could postulate a general level of threat."

The recent memo states, "Drivers must be informed as to the use of deadly force criteria based on federal, state, and local laws."

Burns said that many states forbid drivers to carry arms. Kaine said, "we're not absolutely sure we agree with that," but that all state laws will be obeyed.

No arms shipments have been hijacked, officials said, but recent accidents have caused concern. In 1982, for example, a truck carrying 18 surface-to-air missiles had an accident in California. According to the NTSB, the driver, who pleaded guilty to "reckless driving with alcohol involvement," had been convicted of 14 traffic violations, one count of burglary and two of property theft, and had a robbery charge pending at the time of the accident.

In 1984, a truck carrying torpedoes overturned on a Colorado highway. The NTSB blamed "driver inexperience" for the crash.