A difficult and perhaps impossible task confronts President Carter's two special envoys, who are to arrive Tuesday bent on selling the idea that a pullout of ground forces does not weaken America's pledge to defend South Korea.

The grim prediction of renewed war that led to Maj. Gen. John K. Singlaub's recall to Washington and reassignment has heightened general apprehension that removal of the "tripwire" U.S. infantry division will tempt North Korean aggression.

Under Secretary of State for Political Affairs Philip Habib and Gen. George Brown, chairman of the Joint Chiefs of Staff are to begin consultations with President Park Chung Hee and his ministers Wednesday on the scale and timing of the five-yer phaseout.

Details of the South Korean counter-package are secret, but it is certain to include requests for substantial U.S. assistance to upgrade their military forces and other compensatory measures to offset the military and psychological loss of American ground troops.

Government officials, opposition leaders, military commanders and media opinion-makers here are skeptical and seriously worried at the prospect of a major change in the American underprinnings of this nation's survival.

There is universal confusion about American motives. President Park has said he does not understand the decision, but privately, some officials do not hesitate to link it with Vietnam and Angola as a further example of a weakened U.S. national will.

A recent newspaper column started out: "To begin with the conclusion, the United States should not repeat the mistakes it made just before the start of the Korean War 27 years ago."

The writer, Yu Chino, a leading member of the chief opposition party, then went on to recall the withdrawal of American forces from Korea in 1948 and the late Dean Acheson's 1950 statement putting South Korea outside U.S. defense lines. The Korean War started a few months later.

Virtually no one in South Korea wants the GIs to go home. Sunday, a prayer meeting in Seoul drew approximately 500 Christians - many of them ardent critics of government human-rights violations - who sang "Onward, Christian Soldiers," and prayed for a change of policy in Washington.

"We tried to express our misgivings and uneasiness about the troop withdrawal," said the Rev. Kim Kwan Suk, secretary general of the Korean National Council of Churches. "If it's possible. We would like to reverse the decision."

Government agents blocked advance announcements of the service - apparently in line with a policy to keep the nation calm. Troop withdrawal is seen as a complex issue that could possibly threaten South Korea's national security, its internal stability and an economic well-being heavily reliant on the confidence of foreign investors.

The Park government has recognized the irrevocability of the Carter plan and is taking practical steps to foster "self-reliance" and minimize the domestic impact.

The President himself set the example recently when he regretted the U.S. decision but declared that South Korea's security would remain intact. "I am confident we must defend our country for ourselves," Park told an interviewer. "I am confident that our ground troops can smash the invasion of the North Korean Communists if only we are provided with adequate air, naval and logistic support."

The president noted his concern, however, that if North Korea provoked a war, China and the Soviet Union would "automatically and immediately intervene" on the basic of military pacts.

Reliable U.S. sources insist that President Park and senior South Korean military men are far more seriously troubled than they indicate by the prospect of the departure of the 2d Infantry Division.

"They don't believe it won't destabilize the peninsula," a U.S. officer said. "And I don't think the North Koreans or the Chinese believe it, either."

The business community is also troubled. "The retreat of American troops increases the rick of war," said a corporate lawyer. "With the President and the Congress showing they don't want to help South Korea, we wonder if the American banks will want to keep on lending us money."

The South Korean government has long been preparing for the day when it would contemplate a war without U.S. ground troops. A $1.5 billion U.S. program to modernize the South Koreans project spending a further $5 billion by 1981. Estimates of the extra costs necessary to replace the extremely well-equipped U.S. troops have ranged up to $8 billion.