I don't recall the JAT 737-300s picking up any of the holiday charter work either, they seem to have stayed based at Belgrade on scheduled flights. The first one was delivered in 1985; by this time the Croatian-based holiday industry was already starting to fall out with the Serbian-based JAT from Belgrade, and I get the feeling that JAT pulled right back from Dubrovnik and Pula holiday operations before the overall carrier fell by the wayside for a few years in the 1990s.
Incidentally those 737-300s are still an intact fleet with JAT (they were leased out during the civil war, but all came back afterwards), and nowadays are still daily visitors into Heathrow and similar major points, where they fall among the oldest daily visitors.