It looks as if we may not have bowden cables on the 81 300 D's

Through my experimentation with different levels of vacuum to the modulator on the transmission. I have found if there is no vacuum at all to the transmission I will get one shift from first to second at a very high rpm with a chirp from the back tires, like yours is doing.

The previous owner connected the vacuum line directly to the main vacuum line coming from the vacuum pump. Bypassing the vacuum regulator connected to the top of the IP pump. This gives me very low rpm shifts through all gears with a very light foot on the gas. If I press any harder on the throttle I get flairing ( sounds like the transmission hits neutral )

I hope this at least gives you somewhere to start tuning your transmission.
RichC

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