When I experienced extremely Hash down shifts to the it was because I had accidently disconnected the Vacuum Line at the Transmission Modulator (722.315).

All of My forward shifts seemed only slightly different.

It could be your modulator is not working.
As said before a Hand Held Vacuum Tester a.k.a. Mighty Vac is a valuable Tool For testing Vacuum Leaks and testing Vacuum Components for leaks like the Trans Modulator or the Vacuum Shutoff Valve/Servo on the Fuel Injection pump.

If you are just checking to see if the Lines have Vacuum you can get by with one of those Fuel Pressure/Vacuum Gauge combinations.

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84 300D, 82 Volvo 244Gl Diesel