Slamming clunk a few moments after going back into 2WD

[troverman]

Guys, I have a 2005 GMC 3500 dump truck with the 6.0L and its 4x4 with the manual floor shifter transfer case.

 

Going from 2WD into 4HI is very smooth and it works well. Going from 4x4HI back to 2WD is different. I pull the lever back into 2WD, and within a second or two the 4x4 indicator lamp inside the instrument cluster goes out and the steering is free (I imagine because the transfer case has just disengaged). But then a few moments later, sometimes a minute or so later, there is a big time clunk and the truck shudders. I'm guessing this is the center axle disconnect disengaging? I am shifting in and out while on the move, but never while I'm pressing the accelerator, always just coasting. Is the CAD failing?

[Jim tait]

I have experienced the same issue do you have any answer to your problem.

[davester]

It probably isn't the locking mechanism in the front diff, as typically when that fails/breaks/doesn't work right, it either stays locked (more rare), or just doesn't work and stays unlocked.

 

I would say it's more likely something in the front driveshaft and/or front diff that is unbalanced/worn, and causing your shudder as the assembly slows down when the front axle is unlocked.  I would start with checking the u-joints in the front driveshaft, and then perhaps bearings in front output shaft of the TC, and the bearings in the front diff.

[troverman]

For me, the solution seems to be to make sure I'm stopped when I shift from 4WD back into 2WD. I even put the transmission into neutral. No clunk when doing it this way. Shifting into 4WD on the move never has any clunking, just shifting out. No big deal, I can do it while stopped and in neutral. I can check the driveshaft u-joints and play in the front bearings once the snow is gone. The truck plowed snow all day yesterday without issue, so I'm happy.

[davester]

You might also just try rotating the front driveshaft by hand in both directions, in 2wd mode, and seeing how it feels.  The clunk can be from one part of the assembly slowing down faster than another (say, between the ring and pinion gears).