OTL, Yugoslavia never got to fight a real war, and ethnic tensions remained the biggest sources of division. An ATL Yugoslavia would need at least two PODs to survive to the present day. A major war involved with the state's founding and a Cold War partition that cuts across ethnic lines are necessary to unite the state.
POD 1: We have made Yugoslavia, now we must make the Yugoslavs
A major border conflict with Italy in the years after WW1 provide a unifying force for Yugoslavs. A conflict over the Dalmatia could escalate into a Polish-Soviet war style conflict between Yugoslavia and Italy. In the same way that the Polish-Soviet war zone stretched from Kiev to the outskirts of war, the alt kresy for this conflict would include region from the Adriatic Coast through Croatia and Bosnia up to the outskirts of Sarajevo or Belgrade. A more difficult "war of independence" with Serbs and Croats fighting together against Italians could create a unified Yugoslav identity and serve a similar unifying purpose as the Franco-Prussian and Polish Soviet wars.
POD 2: A communist Yugoslavia and a capitalist Yugoslavia
The divide between West(ern) and East(ern) Germany overshadowed any lingering divide between Catholics and Protestants or Northerners and Southerners. A Yugoslav inner border with a sufficient number of Serbs and Croats on both sides is a necessary POD to A) require both communist and non-communist Yugoslavia to build a "Yugoslav" identity in their respective countries and B) create sufficient support for an eventual Yugoslav "reunification" that once the eastern bloc falls apart
The best intra-Yugoslav border would be a line that roughly follows Sarajevo's latitude. This leaves a non-communist "South Yugoslavia" in Dalmatia, the southern half of Bosnia-Herzegovina, Montenegro, Kosovo, Macedonia, southern Serbia and an eastern-bloc North Yugoslavia in the remainder of the country. This North Yugoslavia could be a large, somewhat viable state yet "digestible" enough to be absorbed by its neighbor GDR-style.
Depending on the details of a partition or occupation zones, the border could be adjusted so Belgrade or Sarajevo play the alt-Berlin role of divided capital, with Bonn analogues in Zagreb and/or Skopje.