I'm curious about this too. I do know there is a service where we can send in our existing differential and it will be modified to an LSD for a charge. This way you retain "stock" parts and the LSD is added. I wonder how the traction control would act as a result?
Do the E420/430/55's have a different gear ratio for the trans than the E300/320? Maybe swapping the trans is another option although much more expensive than the diff.

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Ed
-1984 Mercedes 190D 2.2 5-speed gray market(bought@30,000 miles) (Sold back to original owner@170,000 miles)
-1999 Mercedes E300DT (245,000 miles)
-1999 Mercedes S500 Grand Edition (80,000 miles)