BELGRADE -- One of the least-appreciated wonders of the changes in Eastern Europe in the past year is Yugoslavia's climb back from the economic abyss.

Western economists favor the word "remarkable" in describing what has happened here. Hyper-inflation is dead, exports are surging, tourism has had a banner year and hard-currency reserves have jumped to $10 billion.

In Poland, a similar program of "shock therapy" reform has triggered a crippling 30 percent drop in industrial output. Here the drop has been less painful, around 9 percent.

But just at the point where Yugoslavia's free-market reforms need to make fundamental changes in property ownership and the operation of big business, economists and political analysts say the program has smacked into a brick wall of nationalist rivalry.

Outside of the Soviet Union, Yugoslavia is the most complex, fragile and politically explosive state in Eastern Europe. It has six republics, two provinces, more than a dozen nationalities, six languages, three religions and two alphabets. With the coming of free elections and the eclipse of communism, long-simmering nationalist resentments have begun to boil.

"We don't have much time for economics," said Slavin Letica, senior adviser to the newly elected president of Croatia, which is struggling to deal with its half-million-strong Serbian minority.

"The whole thing {economic reform} is starting to disintegrate," said Vladimir Gligorav, a Serbian economist. "We are left with nothing but nationalism."

These are probably overstatements. The economic reform program initiated by Prime Minister Ante Markovic has succeeded far beyond the expectations of most Yugoslavs. Polls show that the program has made Markovic this country's most popular politician.

Last year, inflation roared away at an annual rate of 2,500 percent. The government frantically printed money in higher and higher denominations. By the time Yugoslavs figured out how much a new dinar note was worth, it was well on its way to being worthless.

Since the implementation of Markovic's economic stabilization program in January, the dinar has become as solid -- at least inside Yugoslavia -- as the German mark. In fact, the value of the dinar has been pegged to the rise and fall of the German currency.

There are a host of economic indicators showing genuine progress for a post-communist country struggling to embrace capitalism. Both imports and exports are up. Since January, hard currency reserves are up $3.5 billion. Foreign investment, recently freed of restrictions, is increasing.

But there is a second stage to the reform program. It calls for wholesale privatization of state-owned industry and a dramatic decrease in government control over banks. It also calls for a radical change in the federal tax structure.

The second phase, demanding systemic change in how business operates, requires that the six autonomous republics bend to the will of the federal government.

As nationalism surges, however, many republics are refusing to support the reform program -- even though it is clearly working.

The two richest and most northerly republics, Slovenia and Croatia, have declared that they will secede from Yugoslavia unless they are guaranteed that this country will become a "confederation." In effect, they want the kind of independence that Belgium or Spain has as a member of the European Community.

Slovenia and Croatia demand sovereignty over their economic and military affairs, and are reluctant to go along with any plan that increases the taxing authority of the federal government.

Those Western-oriented republics, however, have shown considerable interest in Markovic's plan for privatizing state-owned industry.

It is in Serbia, Yugoslavia's largest republic, that opposition to privatization is most unyielding.

Serbia has yet to hold democratic elections and the ruling Serbian Socialist (formerly Communist) Party continues to control many of the republic's largest industries. Tens of thousands of Serbs owe their jobs and their future to the party.

Diplomats and economists here say Serbian President Slobodan Milosevic wants to keep it that way. "Milosevic wants the party in the enterprises. It is his power base," said a Western economist.

Accordingly, the federal government's plan to transform all enterprises into share-holding companies has been challenged by the Serbian Socialist Party. Analysts say the plan has little hope of being implemented unless Milosevic is defeated in a presidential election expected late this year.

As nationalism slows the federal government's economic reforms, there has been what one economist describes as an "ominous" 30 percent increase in wages in the past two months.

A wage freeze in the first half of 1990 was a key component in throttling inflation.