What year did Jeep switch to the serpentine belt on 4.2 liter 258ci inline 6?

Jeep's 4.2L (258ci) inline-6 engine transitioned from a V-belt system to a serpentine belt system in the mid-to-late 1980s. There's no single year it changed across all models; the switch happened gradually, with different model years and possibly even trim levels adopting it at varying times. To know for sure about a specific Jeep, you'd need to check the vehicle's documentation or the engine bay itself.