Check my web site here for what goes where on the 1983 300SD. They vary from year to year - not sure which year you have.

Focus on connections around and on the vacuum control valve on the injection pump (driver's side of the engine) and see if you have any vacuum hoses disconnected. Also check under the car and verify if the vac hose is connected to the transmission, next to the vacuum shift modulator. Your car should also have a vac house routing diagram on the cross piece at the front, visible when you open the hood, on the passenger's side. That will tell you what goes where.

Also check for fractured vac hoses - these get brittle and fragile with age, so just because everything is plugged in doesn't mean one of the hoses isn't broken. I give the part # for replacement hose in my piece.

The tranny is vacuum controlled as regards shifting and a disconnected hose would be consistent with your chronology.