President Reagan has decided to replace Deane R. Hinton, the U.S. ambassador to El Salvador, as part of the president's attempt to shore up his Central American policies by putting "his own people" in the key positions dealing with the region, administration officials said yesterday.

Hinton's impending departure was revealed a day after Reagan ousted Thomas O. Enders as assistant secretary of state for inter-American affairs in a move described by one official as reflecting "unhappiness with the execution of U.S. policy" in Central America.

A senior administration official said last night that Gerald E. Thomas, U.S. ambassador to Guyana, is "a good possibility" to replace Hinton. Another senior official said that Thomas has been recommended for the job by national security affairs adviser William P. Clark and that the decision would soon be before Reagan.

Hinton, a career diplomat, is completing two years as ambassador to El Salvador, whose civil war between leftist guerrillas and the U.S.-backed government is the focal point of U.S. involvement in the region. Hinton is about to begin a two-month leave, and, an administration official said, "he won't be coming back."

Asked about this by Washington Post correspondent Christopher Dickey in San Salvador, Hinton said: "I think that story might be traced in Washington. No comment. I have said all along that I'm ready to serve or to come back. We'll see . . . . I serve my president, our president, where he thinks I can best serve. It's up to him."

The official said the decision to replace Hinton was dictated both by the fact that "he's tired out after two years in a pressure cooker" and by Reagan's desire to staff major Central American policy jobs with people of unswerving loyalty to his ideological view of the region. Hinton was picked for the El Salvador post by Reagan's first secretary of state, Alexander M. Haig Jr.

Administration officials said the original plan called for announcing the replacement of Enders and Hinton later as a "package deal" to make the changes appear to be a routine rotation of diplomatic assignments. But that plan was changed because of concern over premature leaks about Enders' ouster.

Some administration officials yesterday also sought to counter reports quoting other officials as saying that the dropping of Enders signaled a shift toward a tougher, more dogmatically anti-communist stance in Latin America with Clark wresting control over policy from the State Department.

A senior official accompanying Reagan at the Williamsburg summit insisted that Enders' replacement by Langhorne A. (Tony) Motley, U.S. ambassador to Brazil, resulted from conflicts in personality rather than policy. The official, who declined to be identified, said that Secretary of State George P. Shultz would now exercise day-to-day direction over Central American policy. But this was greeted with skepticism by other officials, who said Shultz does not have the time.

Despite the effort to play down Clark's role, well-placed administration sources have made it clear Enders was dropped because he had run afoul of Clark and U.N. Ambassador Jeane J. Kirkpatrick, another influential Reagan adviser.

They, together with other critics in the Pentagon and CIA, had become increasingly dissatisfied with the State Department's assessment of the situation in Central America and believed that Enders, instead of putting priority on a military defeat of the guerrillas in El Salvador, favored a diplomatic solution to the civil war there.

Precisely what policy changes will result from Reagan's reshuffle of personnel is still unclear. But Enders' dismissal underscored anew that Central America remains at the top of the presidential agenda, generating public and political controversy overshadowing even such perennial foreign policy concerns as the Middle East and arms control.

The origins of the current U.S. involvement in the region, long troubled by political instability and social inequality, go back to 1978, when leftist Sandinista guerrillas in Nicaragua toppled an entrenched, U.S.-allied dicatorship and set that country on a Marxist-influenced, Cuban-supported course.

The Nicaraguan revolution, aided by what the Reagan administration contends are massive communist arms support and direction through Cuba, gave fresh impetus to a similar, leftist guerrilla insurrection in El Salvador and threatened to revive still other insurgencies in countries of the isthmus.

But, while Reagan has given top priority to combating the trend through infusions of military aid and the dispatch of U.S. military advisers to El Salvador and Honduras, the Salvadoran civil war has dragged on inconclusively. The situation confronting the administration in the region has these characteristics:

* EL SALVADOR: Despite U.S. aid, the Salvadoran military's fight against the guerrillas has been stalemated, and most experts believe prolonging that situation will lead eventually to a guerrilla takeover. Reagan's proposed solution--more arms and training, including plans to send 100 advisers to a new training base in Honduras--has encountered increasingly stiff opposition from Congress, where there is fear of a growing U.S. involvement reminiscent of Vietnam.

Politically, plans have been made for elections at year's end that would carry forward El Salvador's transition from military dicatorship to civilian rule. But the Salvadoran left's refusal to participate has caused widespread charges that the elections will be an unrepresentative sham giving power to the right, that human rights abuses continue unabated, and that the only solution is through the kind of negotiations with the guerrillas that the Reagan administration opposes so vehemently.

* NICARAGUA: U.S. efforts to choke off the flow of arms from the Sandinista regime in Nicaragua to the leftist guerrillas in El Salvador have brought Reagan to the edge of confrontation with Congress over revelations that the administration has been funding covertly an anti-Sandinista guerrilla movement of about 7,000 men with uncertain U.S. control.

The administration has contended that the guerrillas represent a backlash against the "leftist tyranny" of the Sandinistas, and it has denied that U.S. efforts are intended to overthrow the Nicaraguan government. But the situation has exposed the administration to charges that it is trying to turn the clock back to the days of rightist dictatorship, and there is increasing pressure in Congress to put brakes on the Nicaraguan "secret war."

* GUATEMALA: The largest country of Central America also is grappling with a guerrilla insurgency, and the Reagan administration believes a renewal of U.S. military aid and cooperation is vital to keep Guatemala from eventual communist control. But despite Reagan's efforts to portray the new military government there as moving toward democracy, continued human rights abuses by the army in the countryside have blocked any immediate hopes that Congress might approve security aid.

The situation in these countries threatens to spill over into the other Central American nations: Costa Rica, a democracy that has no army and is hard-pressed economically; Honduras, whose increasing military cooperation with the United States could lead to open conflict with Nicaragua and boomerang in a swing of public sentiment to the left; and Panama, whose military rulers must cater constantly to a strong leftist and anti-American sentiment.

Many outside experts have argued increasingly that the countries of the region must get together to negotiate a solution to these tensions. So far, however, efforts to do so have failed. Given Washington's emphasis on defeating the leftist forces by military means, there is a big question about whether the administration would support such an effort.