well runaway diesel got my attention!!!!

my first question is how are your brakes??? Is the pedal harder to push to get the car to stop? One or two leaking vacum elements usually wont cause the engine NOT to shut off.

Does your HVAC controls work? They too are controlled by vacum on this car. Everyone who owns a Mercedes of this vintage or older pretty much buys a mity vac as a matter of fact since they ALL have vacum issues. It is as handy a tool as a regular or philips head screw driver for these cars.

If your brakes are harder to push to get car to stop.. I'd start with the vacum pump as a potential problem. How many miles on your TD?

The modulator line going to the trans should not disable the engine shut off either. Welcome to the world of the Mercedes vacum!!!