The 4 wheel drive on a 2002 trailblazer will not engage, replaced the actuator

SOURCE: 4 wheel drive will not engage - how do I troubleshoot

The vacuum switch on the transfercase is most likely the problem. If you remove it and the plunger is stuck in, replace it and your 4wd should engage again.

SOURCE: My 1999 chevy silverado 1500 four wheel drive

You probably have a bad encoder motor.It is located on the side of the transfer case on the drivers side.It is black.The other problem they had with these was the switch but you have already replaced this so your next thing is the encoder motor.Hope this helps.Good luck.

SOURCE: My 2002 2500HD 4x4 front axle not engaging

Try jacking up one side of the front and putting it on a jackstand. Then turn key on(but dont start it) and put it in 4hi. Try to spin the tire in the air. The drive shaft should not turn nor the wheel. If the wheel does then maybe the acuator is not moving in the axle. It worked when you pulled it out but it might be hanging up on something in the axle.

SOURCE: 2005 Trailblazer no 4 wheel drive.

usually the actuator will still make the light still come on if its bad I would check the actuator

SOURCE: 2002 trailblazer making loud humming noise when driving

When you turn right - you're taking weight off the right wheel.
If the noise dissipates - the passenger side wheel bearing is most likely to blame.

Thanks

Testimonial: "thanks, wish we would have known that before my husband spent 3 hours on it yesterday..it does make sense tho..thank you VERY much for your help"