Rear clunking sound with first snowstorm...

[rob_duvalle@co.washington.or.us]

Portland just got our first snow last night and a chance to really test out 
the diff locks again..... I have an 87 4000QCS, after driving home (approx. 
20miles), upon slowing down I noticed a repeated mechanical clunking sound 
from the rear of the car (differential?).  It would increase and decrease with 
different speed levels.  I disengaged the font diff lock and it continued for 
another 10 minutes and then went away.... Anyone have any thoughts on what to 
check out?  On the way to work this morning it again came back and then 
dissappeared after disengaging the center and another 10 minutes.  I have 
changed out the front and rear diff fluid approx 2K miles ago with synthetic 
in front and standard in back.

Doesn't make sense to me because all 4 wheels are powered all the time so, if 
there was a mechanical problem in one of the diffs, it should show up all the 
time, right?  And if it is a problem with the rear, why does it only start 
when I engage the center?

Thanks for the help....

Rob DuValle rob_duvalle@co.washington.or.us
	    rduvalle@onlinemac.com
87 4000QC