Clunk Noise from 2nd to 3rd

There is a small bushing that is located right in the center of the rear of the transaxle/differential (thing in the center of the two
rear wheels if you look underneath the rear bumper) that is worn. It is just metal to metal there. It causes loud and vicious clunk if I let the car shift from 2nd to 3rd at 23 - 29 MPH. Otherwise it is pretty smooth and silent.
The mechanic proved to me it is that bushing that is worn by putting the car on the lift, jacking up the differential (or whatever that thing is) with a vertical plank of wood and jamming some pieces of rubber in where that bushing would go. There was no disassembly required. It is perfectly visible from just peering underneath the car there that there is a 2" circular metal protrusion coming through a flat piece of metal in the rear epicenter of the suspension with a flat piece of metal in front of it with a 2" hole for the protrusion to come through. It quieted the sound somewhat.
Somebody a while ago said it was probably the "FLEX DISC(S)" causing this clunk before I knew where it was coming from. Is this thing the flex disc that I am describing?
The mechanic doesn't know what's involved in replacing that bushing. Does anyone else know the process to replace it and how long it should take?
'87 Mercedes 300D Turbo (W124 chassis, unknown mileage)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.