Key personnel at “Dateline: NBC” made serious judgment errors in conceiving, reporting on and responding to attacks on its story on General Motors’ trucks, according to an internal report from NBC News released yesterday.Three of those involved — executive producer Jeff Diamond, his No. 2 David Rummel and the producer of the segment Robert Read — officially resigned yesterday. Neil Shapiro, who came to NBC from ABC’s “Primetime Live,” has been named to replace Diamond. Michele Gillen, the correspondent on the “Dateline” segment that alleged GM trucks had a tendency to explode on impact, has been reassigned to an anchor position at the NBC owned-and-operated station WTVJ, Miami. Gillen appears to have escaped a forced resignation by convincing investigators and NBC honchos that she had objections to the methodology used in the controversial crash early on. “These journalistic and administrative failures are indefensible,” NBC chairman Robert Wright said in a statement released with the report. “They should not have happened. And they must not happen again at NBC News.” Prepared by attorneys Robert S. Warren, of Gibson, Dunn & Crutcher and Lewis B. Kaden of Davis Polk & Wardwell, the inquiry focuses on several instances where the “Dateline”/GM piece went awry. The report charges no deliberate attempt was made by those involved to deceive, but found there were “serious flaws” in judgment that led to the debacle that has rocked the network since the automaker attacked the “Dateline” segment in a Feb. 8 news conference. The investigators found the crash test was ill-conceived in its purpose and was carried out cheaply and amateurishly. They charge that the ensuing piece, which juxtaposed other test-crash footage with the NBC-commissioned test footage that only produced a small fire, undermined the segment’s accuracy and fairness. The investigators also charge that “Dateline” producers were remiss in not revealing that rocket igniters were placed underneath the test trucks, although the report tends to suggest they had nothing to do with starting the fire. The report also found fault with how NBC handled early complaints by GM about the “Dateline” segment, charging there had been insufficient investigation early on. In a statement released with the report announcing the resignation of Diamond and Rummel, both are carefully absolved of “rigging the test results or involved in any conduct intended to present false information.” Sources close to Diamond and Rummel said the carefully worded language of the release was part of an agreement reached that “imposes confidentiality.” The announcement of Read’s release contained no such language. “I continue to stand by the accuracy of the report,” Read said in a prepared statement. “I remain proud that I helped bring this important debate about the safety of certain GM trucks to the public’s attention.” Meanwhile, staffers at “Dateline” began to pick up the pieces from the debacle. Wright met with “Dateline” staffers yesterday, and according to the newsmag’s anchor Stone Phillips, reassured the staff of the network’s commitment to ‘Dateline’ and encouraged them not to back away from investigative reports.