1995 E320 coupe - Transmission clunk on 3-4 upshift

[bigwalt]

I just had a new problem develop with my coupe. I am getting a loud clunk back at the rear differential area when transmission upshifts from 3 to 4. Clunk actually sounds like it is coming from the trunk area. It is very pronounced on a light throttle upshift - but if I accelerate under heavy throttle I don't hear the clunk when it upshifts from 3 to 4. The upshifts also seems a little harsher than they have been in the past. I'm suspecting it might be related to a vacuum leak or vacuum modulator problem ? Other than the clunk - transmission seems to be fine - no slipping or other noises. Any idea of what cause might be??