TWENTY YEARS AGO John Howard Griffin, a white Texan, dyed his skin to disguise himself as a Negro and travel America's underside. He described his tour in a book and movie, black Like Me . Griffin's willingness to have done unto him what few white Christians would admit to doing helped show the extend of local tarnish on the general Golden Rule.

Now Chet Fuller, a native Atlantan newsman born the right color, has made a similar journey to evaluate changes in the lives of poor blacks in five southeastern states. Home again in his suburban, middle-class neighborhood, he draws the same conclusion as Hosea Williams -- then president of SCLC -- who told him in Gadsden, Alabama, "They've thrown off one false face and put on another. What New South? What New South?"

A college graduate and winner of newspaper awards who for this assignment needed "to be handkerchief-headed" around whites but cool and streetwise around blacks, Fuller set forth wearing workclothes to hunt jobs in large and small cities, to stay with poor families in rural shanties and crowded neighborhoods. One weakness of his plan: the chosen person of a drifter without local references or job experience would be no asset even to a white man seeking employment in the non-thriving communities he visited.

Also, unlike Griffin, he frequently abandoned pretense and admitted to being a reporter with notepad, camera and tape recorder. The obvious results of this were his 10-part Atlanta Journal series syndicated in 500 other newspapers a spot on the "Today" show, this book, and a $14,000 Charlotte OBSERVER found for one of the underprivileged North Carolina families he wrote about. Less obvious is whatever possible magnetic effect the promise of publicity had on these already lopsided interviews.

Fuller's account, though, never claims to be evenhanded or objective. This is personal testimonial, even polemic. When blacks in Bladen County, North Carolina, say a state officeholder did thus-and-such injustice to his tenants, he is given no chance to reply. In one Mississippi town, Fuller hears that a black man has been gunned down by a white one who stopped his pickup at a traffic light, and that "police there knew who did it, but refused to do anything," and the hearsay stands, sans verification or denial. Or Eddie Lee Mullins, 16, Americus, George, who has arleady served seven months in prison because, as he says, he "stole this white man's car," is interviewd as solemnly as any expert on American society. Only the victims testify, and their stories are sad.

Perhaps his editors, wearying of the persistent victimization, requested the afterthought chapter of moderately good news that Fuller made a second trip to get: a humane, nonprofit housing development for the poor in Americus, which Millard Fuller supervises for Koinonia, a nearby Christian farming co-op; a CETA project in Kingville, North Carolina, where Marcia Kuhn helps unskilled get and keep good jobs; and the success of Delta Enterprises, of Greenville, Mississippi, where Harlem native Charles Bannerman heads a black financial empire with 1978 revenues of $7.5 million.

All these are second-look stores. Fuller's first look, he admits, was influenced by heritage of understandable fear from hearing stories such as one about signs posted along Alabama's borders said to say, "Read this, niggers, and run. And if you can't read run anyway." The spirit of that story is still true in much of the South, his journey convinces him.

Fuller's book makes two main contributions which outweigh his tendency to oversimplify. Experiences in North Carolina, South Carolina, Georgia, Alabama and Mississippi show clearly that despite the war on poverty, many blacks are still in dire straits. The victimization of many is real, whether or not in this particualr book victimizers are given equal time. Even in Atlanta where all the city leaders are now black and 60 percent of the population is black, that population holds only 3 percent of the wealth. "Economics is money is power is white . . . . Whites . . . still have the power to strangle us or let us breathe," Fuller writes angrily.

The book's second contribution seems almost inadvertent. Along teh way put with touching honesty, Fuller reveals the deep guilt felt by black professionals like himself, now in "the mezzanine of the American social structure," who confront "life at the lower end of the American Dream," knowing how narrowly they have escaped being there. His resulting pangs of consience go twice as deep as those Griffin felt in his book, for this brotherhood is not only that of fellow man, but of fellow black man.

While part of the "name" Fuller hears in his title must be the stereotyped call for "old Black Joe" still sounding, some of the gentle voices he hears are more universal -- those of the field hands left behind when one of them moved "up" to the big house, the larger cry of the unlucky which always rings in the ears of hte fortunate unless they deafen themselves, the weary sigh of the old after the strong young. Part of the "calling" Fuller hears is the inner conscience of good men of all colors when confronted by injustice and pain.

The reader's shared gult agrees with Fuller that instead of "sitting around lamenting the hugeness of the problem, you do whatever you can." As for the book's final sentence -- "And because there is always God, there is always hope" -- well, yes.

Yes, but.

But God must weary of being so easily invoked rather than having that Golden Rule obeyed.