Where is the voltage regulator located on a 91 Jeep Wrangler 4.0?

On a 1991 Jeep Wrangler with the 4.0L engine, the voltage regulator is internal to the alternator. Unlike some older vehicles where it's a separate component, the 91 Wrangler's voltage regulation is integrated into the alternator itself. You won't find a separate voltage regulator to replace. If you're having charging issues, the entire alternator typically needs replacement.