99 Blazer - 4x4 engages and disengages but not front axle
 

This happens just about every year and it's time to get it fixed. I have a 99 Blazer 4x4 automatic. Has 2WD Hi, 4WD Hi, 4WD Lo and AWD buttons.

What happens:
Put it in 4WD Hi and 4x4 engages. When I go back to 2WD Hi the 4x4 disengages (I know b/c I can spin the rear tires in the snow and front don't move). The light on 2WD Hi button comes on but feels like the front axle is still engaged. The front end feels really tight and like I'm driving in second gear.. not getting as much roll.

What I try to disengage:
I then try to go back to 4WD Hi.. and it works (button light comes on). Then I try going back to 2WD Hi, 4WD disengages, 2WD Hi button lights and "Service 4WD" light comes on. Pushing any of the 4WD buttons does nothing from then on out. When I turn the car off and turn back on the service light goes out but the front end axle still feels engaged.

Is this a vacume or actuator issue?

Thanks for your help!

Happy Holidays,
John