4x4 engages while driving

[boutbroke]

My grandson bought a used 1989 (160K miles) Blazer. The 4WD works fine but sometimes the light comes on while he is driving it. I seem to remember the 4x4 was engaged using a vacuum. Is this true and if so did the vacuum hold the transfer case in two wheel drive? I'm suspecting a vacuum leak. It does have the shift lever in the floor. When it engages he moves the lever back and forth to clear it. Could it be in the mechanical linkage?

As you can tell I don't have a lot of knowledge about this so I would appreciate any help.

[blue2kzr2]

If it's just the diagram of the front axle that lights up, it's the vacuum switch on the t-case. It's tough to get to but nothing terribly hard to do for someone that can do an oil change...

[boutbroke]

Thanks,

More info, he says he can feel it physically shift in and hear the gears.