GMC Jimmy Electric Shift 4 Wheel Drive

I have a 2000 GMC Jimmy with 100,000 miles. When I press the buttons on the dash to shift to 4 Wheel High or 4 Wheel Low, the button lights change, I hear the solenoids click, but the front wheels receive no
power. It shifts in to low range when I press 4 Wheel Low, but again no power to the front wheels. What is the most probable cause? I have heard others complain about the push-button 4 Wheel Drive having probles at about 100,000 miles.
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When you press the button and change into 4hi or 4lo, you can still have problems if the front axle is not engaging. Some diagnostic work needs to be performed to see if you are getting vacuum down to the vacuum motor that engages the front axle. If in fact you are getting vacuum down there, then the only other option is a mechanical failure within the front axle disconnect system.
Ian
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