Can the solenoid be replaced without removing starter on a 1998 Plymouth Voyager minivan with 3 liter engine?

No, on a 1998 Plymouth Voyager with a 3.0L engine, you generally cannot replace the solenoid without removing the starter. The solenoid is an integral part of the starter motor itself. While access might vary slightly depending on the exact configuration, the starter typically needs to be removed to access and replace the solenoid. Trying to replace it in place would be extremely difficult and likely impossible without damaging components.