IF THERE IS any moral force behind the cause of striking independent truckers who are protesting new highway taxes, it is fast being dissipated by hundreds of acts of violence on the nation's highways. By Thursday morning, the fourth day of the strike, 163 trucks had been hit by gunfire and 167 by rocks and other objects. One trucker has already been killed and a 14-year-old Pennsylvania girl was hit in the head with a brick.

Federal officials claim that only 20 percent of the independent truckers joined the strike voluntarily, but as the violence increases--euphemistically described by one strike leader as a "tax war"--others have become too frightened to drive. Shortages of produce have begun to occur in the Northeast. Those drivers who want to continue to work--many have no alternative if they want to keep their trucks and feed their families--have limited nighttime driving, avoided major interstate highways and organized convoys for self-protection. Whose responsibility is it to help and protect them?

State police are doing what they can. In eastern states, where most of the violence has occurred, highway patrols have been beefed up and surveillance increased. Escort service has been provided to convoys, and highway patrolmen are working longer hours. But if the violence continues, the federal government will have to step in. This terror cannot be permitted to go on. The FBI is already investigating the killing that occurred Monday in Georgia, and the Justice Department has been asked by the American Trucking Association to consider a range of federal statutes that may have been violated-- firearms violations, transportation of explosives, and the like..

Independent truckers have had to cope with the changes brought by deregulation and many have suffered economically. But this new tax may not be the blow some of them believe. It does not even go into effect until July 1985, and the same law that mandated the tax created an advisory commission to study alternative ways of raising this money. There is, therefore, a chance that the tax will be changed before it is ever collected. Meanwhile, falling fuel prices will offset some of the new burden. Taxes on inner tubes, truck parts and lubricating oil have recently been reduced.

Everyone wants to settle this strike quickly, to keep the trucks rolling and to end the violence, which transforms an economic and political protest into crime. The best course for the truckers is to negotiate with the new advisory committee on alternative taxes. They have 21/2 years to make their case peacefully. The longer the violence continues, the less likely they are to get a sympathetic hearing from lawmakers and the public.