4x4 making crunching sound

[Dennis22]

I have a 96 Burb. w/ 160,000 miles. when I put it into 4X4 I get a banging sound, it is more pronounced when I turn the wheel.It really sounds like something is going to break in the front end. I plow with this truck, when I back up and let off the throttle and coast to a stop it gives me a rythmic banging sound. When it is not in 4X4 it sounds and runs great (except for the ocasional stalling). What turns in the front end only when you hit the button for 4X4? Does the front axles or CV joints only turn when 4x4? Any help would be greatly appreciated.
Dennis

[markrinker]

No. Your axles and CV joints turn continuously.

Check the u-joints on the propellor shaft, as it only turns when 4WD is engaged. (Short drive shaft running from transfer case to front differential.)

I plow with two 1994's and have replaced propellor shaft u-joints on both trucks #1 and #2 after first noticing noises like you describe.

If you ignore it, you'll end up plowing your own front driveshaft into a snowdrift, and have to replace it altogether...don't ask how I know this...

[diesel65]

There is a possiblity that the snap ring on the rear output shaft of the transfer case is broken.
To inspect you need to remove rear drive shaft and then remove the tailshaft housing that is held by four bolts.

[Dennis22]

Mark, you were right! 4:30 this morning while plowing I hear the big bang! I got out and there is my U-Joint lying on the ground. It is in the shop getting replaced.
Thanks,
Dennis

[Dennis22]

Update: Located drive shaft at local Junk yard. When the drive shaft came loose it also took out the wiring harness for the tranny. I am having a hard time locating one at the local GM dealers. any ideas?