Service 4wd Light When Wet

When driving in wet conditions, the Service 4wd light sometime comes on. It also comes on when I get the under-car wash option at the drive-through carwash. Any idea?

Thanks - HC

Year and transfer case type would help.
If a 236 transfer case a good transmission/engine capable scanner will give you codes.
If any codes post back codes.
If only acting up when wet also check wiring and sensors.
MT