First thing's first... are you sure there's no air in the system?

I also thought ASD didn't lock the diff over 25 MPH.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

I opened a bleeder nipple back at the rear diff while the engine was running and the hydraulic oil came right out. If there was any air in there it wasn't much.

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dieseleverything

'95 E300D 279k 717.411 5-speed swap 40 MPG
'78 240D Turbo 144k
'83 240D 201k