Hello I have an odd issue and before I spend tons of money on a diagnosis.. I'd like to get a direction where this issue is.

1997 Blazer 4.3 automatic drives fine in 2wd.... When engaging in 4x4 the vehicle will start having a knock under the drivers seat that becomes faster with acceleration and over 30mph the vehicle starts to vibrate considerably......

This issue still occurs after putting the vehicle back into 2wd (push button method). After the vehicle sits over night, the issue appears to resolve itself... that is until I put it back into 4x4 then the same exact issue occurs each and every time. I need to let the car sit overnight after selecting 2 wheel drive for it to resolve itself.

Any thoughts?

 

It sounds like you have a faulty u-joint in the front driveshaft. I would pull the front driveshaft and inspect the joints.

It also sounds like you have something amiss with the front axle engagement mechanism that results in slow disengagement of the front axle. This can be caused by transmission/transfer case fluid getting into the lines. I would disconnect the vacuum lines at the vacuum switch located on top of the transfer case near the front output. See if there is any transmission fluid in the lines. If there is, pull the fill plug on the back of the transfer case. If fluid pours out, you will need to drop the transfer case to change the input seal. Regardless of whether or not the transfer case is over full on fluid, the vacuum switch would need to be replaced if there is fluid getting into the lines.

If fluid is in the lines, you need to determine how far it went and get the lines and any other affected vacuum powered components cleaned or replaced. Fluid can make its way all the way up into the HVAC system which can be a royal pain to get cleaned up properly.

 

Maybe front driveshaft if you have 3 button, could be alot of things, maybe your diff is only partially engaging so could be vacuum issue could possibly try putting it in 4wd and giving the cable a good yank with some pliers to see if it goes away then that would be a vacuum issue/actuator/vacuum switch. For some reason id lean towards vacuum actuator because maybe as it sits all night it looses enough vacuum to release but it works just good enough to partially engage front diff. Wow i took along time to type that i didnt mean to repeat the guy above me.