The key to the vacuum shift "klunk" is that it occurs while slowing with ones foot off the accelerator. The problem is really independent of other play in the drivetrain.
The problem occurs because the vacuum for one reason or other drops and shifts are made under pressures that normally should only occur at full throttle. It can easily be diagnosed by monitoring the vacuum to the modulator. With no throttle the vacuum should be above 10 inches AT LEAST. Properly functioning it will go over 20 inches. The shift becomes objectionable at vacuums levels below 5 inches.
Actual play in the drivetrain is more likely to be felt when changing driveshaft rotation: reverse to drive. Usually a bad u-joint will become a vibration long before it becomes a "klunk".
Bosch Master, ASE Master, L1
33 years MB technician