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Q: The four wheel drive system is not engaging the front wheels

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I have a 2004 chevy trailblazer LS 4.2 I6. I don't know what is wrong with my 4x4. The switch works, I checked the transfer case shift motor and it works, I tried replacing the actuator and still nothing. Everything seems to sound fine, The system sounds like everything is moving proper, But still no power to the front wheels. When I Engage into neutral and roll into 4lo i hear/feel it engage, but no power to the front wheels, when i roll out of 4lo into 4hi i hear a loud clank, like if it moved out, or metal hitting metal. Also when i drive with the 4lo engaged i hear a humming noise and it gets louder the faster my car moves. Almost like a winding sound. The next thing i am thinking is the 4wd disconnect housing. I hear the shift forks break within these, but its a expensive part that i don't know if i should try this. Any thoughts?

My car has 170000 miles.
My car has an automatic transmission.

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