Had the rear end of the car jacked up while you changed the oil in the diffy? How'd the rear brake hoses look? How does the brake pedal feel? Could, potentially, have lost brake fluid through the brake system.... Those rear hoses often fail where the fitting that screws into the caliper is crimped on, leading to fluid loss.

Jay

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On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er