I'd find an independent MB mechanic to do the job. You should find a rebuilt axle for $300-400 bucks and expect to pay a couple hundred for the installation. Speaking from experience, I've had them replaced on my 300D and it's pretty much the same axle and same repair.

One other suggestion is that you find an SL being parted out on eBay and contact the individual to see what he/she might charge for a used axle off one of the vehicles. FYI, I picked up a pair of axles awhile back from an SL with 56,000 miles on it for a little more than $50.00. They're cleaned, boots lubricated, and sitting in my attic for the time I need to replace them on the SL.

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000