As shaken U.S. Postal Service officials gathered information on the latest shootings in Michigan and California, some psychologists described much of postal work Friday as a treadmill of angry monotony, with labor-management hostility making many post offices minefields of carefully nurtured grievances.

In California Thursday a postal worker killed a letter carrier and wounded a clerk, and in Michigan just hours earlier another postal employee shot and killed one person and wounded two others - bringing to 29 the number of supervisors and colleagues who have been slain by disgruntled employees over the past decade."The whole system seems to have caved in," said Roger G. Wittrup, a police psychologist specializing in post-traumatic stress disorder, who was called in after four postal workers were killed by a 31-year-old letter carrier in Royal Oak, Mich., in November 1991.

"Someone, everyone, is clinging to the old system," Wittrup added, using the vernacular of labor-management disputes, in which workers make their complaints before hearing examiners and each side files reams of demands. "Somehow we have to realize the solution is sitting down together - burying the ax. This is not just a matter of politeness. It amounts to an attitude check, at all levels."

According to Dan Mihalco, a spokesman for the Postal Inspection Service, a division of the Postal Service, there were 396 assaults by postal employees on their co-workers or supervisors in fiscal 1992, down from 403 in 1991 and 422 in 1990. In the first six months of fiscal 1993, there have been 177 such assaults.

The Postal Service's militaristic tradition, which Postmaster General Marvin Runyon has said he wants to dismantle, can feed the anger of those predisposed to be angry, Wittrup and other experts said.

But another police psychologist, Michael Mantell of San Diego, said that the postal service was the victim, not the villain, in these incidents. "Remember that the third-leading cause of work-related death is homicide," Mantell said.

"There are lots of tough jobs out there," he added. "Let's not become hysterical."

Nonetheless, he said the Postal Service, which has 750,000 employees, more than any other civilian labor force, needed "much much better psychological screening, better training for first-line supervisors and a serious program of psychological counseling."

In addition, former postmaster general Anthony Frank noted that more than 10 percent of the postal work force had been hired in part because they received extra consideration as disabled veterans.

"When you mandate that - and the disability can be mental as well as physical - in a tiny, tiny minority of cases you're going to have people slip through who are basically unbalanced people trained to kill," Frank said. "It's a lousy thing to say but I think it needs to be said."

While Frank and the psychologists and workplace stress experts who have looked into postal violence emphasize the difficulty in making generalizations about such a sprawling institution, they said there must be common threads to link these incidents, far separated in time and place.

In Orange County, Calif., Friday, the authorities were still searching for Mark Richard Hilbun, 38, the dismissed postal worker suspected in the killing of a letter carrier Thursday. Hilbun, who was last seen wearing a T-shirt marked "Psycho," had been dismissed last year after refusing to stop stalking a co-worker.

His lawyer, Donald Glenn Rubright of Santa Ana, Calif., said Friday that Hilbun had "decompensated mentally" in 1992, being charged at one point with drunken driving and assaulting a police officer. After he started acting strangely at work and harassing a female co-worker with constant phone calls, he was briefly hospitalized for psychiatric observation and dismissed from the Postal Service.

In Dearborn, Mich., Larry Jasion, 45, a postal mechanic, apparently shot himself Thursday after killing a co-worker and wounding a supervisor as well as a woman who had gotten the job he wanted. People at the shooting scene told The Detroit Free Press that before he opened fire, Jasion shouted, "It's time to educate the supervisors."