If it helps to know, my 84 300D non turbo has a trans meant for a turbo in it from new. I believe the main differences would be the trans cooler lines in the non turbo are smaller. The later trans have different setups for their bowden cable connection to the throttle linkage. The inhibitor switch may be different as well.
I would try adjusting the modulator until the other changes were becoming harsh, if that didnt fix it then swap the trans out.
A 300k mile trans may be on its way out as well.
If you have access to a good second hand trans that would be a safer route than trying to rebuild yours unless you have rebuilt auto trans in the past.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving