The vacuum to be concerned with is that sent to the modulator. It has nothing in common with other systems except its source. If you tee to this line at its rubber connection near the oil filter housing you should see high vacuum anytime the throttle is at idle position. The relation of this vacuum to your clunk should be evident.
I think all turbo diesel differentials had the 3.07 ratio except the 85 models which went to 2.88, if memory serves me.
While installation is rather simple with no special tools necessary, I always recommend a good independent for value. My techs can exchange one of those differentials in less than two hours which makes it easy for us to only charge the warranty flat rate which I am guessing to be close to 4 hours. There are a couple tricks that facilitate such quick efforts and without experience a general shop might spend half a day and expect more for working on an "exotic".
[Edited by stevebfl on 01-01-2001 at 11:44 AM]
Bosch Master, ASE Master, L1
33 years MB technician