Where is the voltage regulator on a 1990 Dodge W250 4X4?

The voltage regulator on a 1990 Dodge W250 4x4 is integrated into the alternator. Unlike older vehicles where it was a separate component, Dodge (and most manufacturers) by 1990 had moved to internally regulated alternators. Therefore, you won't find a separate voltage regulator to replace. If you're having charging problems, the entire alternator needs to be tested or replaced.