You really don't want to have to pull the 240 rear, install it, remove it, install it back in the 240, install another rear...etc You should be able to find a good diff for less than $50, I even got one for free once.

Bargain as though it needs a transmission...offer $300 or so. No way a non-running, driving car should be $900...

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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