98 Blazer 4x4 Issue

[Truckedup]

I have a 98 Blazer 4x4 with a 5 spd.it's in decent shape,125,000 miles on it.Every once in awhile,sometimes not for months ,it makes a vibration and rapid thumpng noise from right front when in 2 wheel drive.It's like running over those warning strips cut into pavement on the edge of the road.When it happens,it starts as you take off,lasts for a few seconds,then disappears.All together this happened maybe 10 times in 2 years.Coming back from a road trip today I slowed to exit the hiway,there was the noise, and it didn't go away.I pulled over,checked out the right front,wheel ,driveshaft and front diff , didn't feel hot,no oil dripping,the CV joint boot is intact.I continue on,it's real noisy ,not so much vibration but the rabid thumping noise and hissing in the background,worse when going slow,lessens when you accelerate,but doesn't disappear.Stepping on the brakes doesn't change the noise,the brakes feel normal.Turning the wheels to the left or right while driving doesn't change the sound.....but,pushing the 4 wheel drive button makes the sound and vibration go away.Only to be replaced by a feeling that the something is engaging and disengaging on it's own about every time the tire turns twice.Plus the small led's on the pushbuttons for 2 and 4 wheel drive where all out,not lit.Then it was stuck in 4 whl low,thump,thump thump as you take off.Shut it down,restart,play with the button,finally back into 2 wheel high.

Limped down the road for 20 miles to my house. Jacked up the front,spun the wheels,nothing unusual to see or haear,spun the front driveshaft by hand,nothing unusual.Then I started the engine,hand rotated the drivshaft,I could hear and feel the noise. The 4x4 pushbuttons do nothing,the 2 wheel drive led is always on.This sounds expensive.............

[Truckedup]

To maybe answer my own question; I been thinking all along there's an issue with the 4 wheel drive controls.But now I'm thinking it's not that at all but..............The front axle lock up mechanical parts have failed,the sensor to detect full engagment of the axle lock has shut down the system and only 2 wheeel drive is available so the 4x4 pushbuttons are locked out so to speak.

I checked the vacuum motor under the battery using a vacuum pump,it doesn't leak,the cable moves,but who knows what moves inside the axle housing? Seems like it's partially engaged,that's the noise and vibration perhaps.

Might be time to pull the right side front axle tube abd have a look at the lock up fork and gear?