Somebody correct me if I'm wrong, but I don't think a W123 differential can leak unless it is seriously overfilled. It only carries an inch or so of fluid in the bottom, which is splashed up to lube the gears. The static fluid level should not come up as high as the axleshafts. That's certainly what I saw when I had mine apart replacing the axleshafts last month.

Natalie the more I hear about your mechanic, the more suspicious I become. The auxilliary fan has two functions. One is to protec the AC system from pressure that is too high, and that function is activated by a switch on the receiver/drier. The other function is to provide additional airflow when the coolant temperature rises to about 105 or 110C. There are several items in the circuit that can cause failure of the fan, and the receiver/drier switch is definitely NOT the place to start! Has the mechanic looked at a wiring diagram?

And I think your're right about the bouncing coming from the rear accumulators and not the front shocks. Search here for the method to test them.

__________________
Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References