have a 2001 jimmy when selecting 4hi lights on switch flash. have heard a humming noise from front off and on. could this be 4x4 actuator valve causeing this or ?????????????? please help. does not shift into 4x4 please help....

After you check the easy stuff. Check your encoder. It is on the tranfer case, eletronic shifter.

The Electronic Transfer case Case Encoder motor is defective.

Swap a working one in from a wreck in a pick and pull and you should be set.