AFAIK everybody who has done the swap has some vibration around that rpm range. The 300Ds that came with manuals in had some sort of extra disk between the flex disk and the transmission that may have been used to eliminate this vibration.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine