First--- Find a new dealer.
The 126 has rear subframe bushings, trailing arm bushings and maybe a hydraulic leveling suspension.
The rubber in the car may have deteriated and need replacing.
In the rear you have either sub frame bushings, trailing arm bushings, differential mount rubber, and sway bar connections and rubber swaybar mounts. OR you have the subframe bushings, trailing arm bushings, sway bar mount rubber and connectors and a hydraulic shock leveling system. You will have to determine which your car has.
If you have just a rubber/shock absorber system look here. This is the rebuild on the 300TD I have. I did the front and rear. Both can effect the ride. Mercedes used basically the same setup in the 123 and 126 for suspension design, the only big difference was the track mount setup in the front suspension.
http://www.peachparts.com/shopforum/showthread.php?threadid=60560&highlight=300TD+suspension+rebuild
rear rebuild
http://www.peachparts.com/shopforum/showthread.php?threadid=60964&highlight=300TD+suspension+rebuild
Go here to see a diagram of your rear suspension
http://www.************************/BitMapPage.aspx?bitmapid=149&bitmapname=Rear+Suspension
Item #5 sub frame bushings
Item #4 trailing arm bushings
item # 2 sway bar connectors
Item # 1 shocks
Go here to see the hydraulic leveling system, You will have the same rubber system but your shocks are hydraulic not gas filled stand alone units. The hydraulic system has actuators ( shocks), control valve, 2 accumulators, an engine pump and resevoir, and lines to the rear system.
http://www.************************/BitMapPage.aspx?bitmapid=151&bitmapname=Rear+Hydraulic+Suspension
Its not a diagram but lists the parts.
Item #1 accumulators, act like shock absorbers
item #4 hydropnuematic shock replaces a shock absorber, but usually don't fail and are expensive.
Here is your front suspension
http://www.************************/BitMapPage.aspx?bitmapid=150&bitmapname=Front+Suspension+%26+Steering
The items that can effect your ride are #'s
1,4,5,7,13,16
The other items are probably not effecting ride as much as they would effect steering
So yes you do have subframe mounts.
Dave