Replacing diff mounts is no fun but $1000 is steep. Your transmission could quit before you retire your car but not from leaking a quart a year. The question is whether you should throw $2000 into rebuilding the rear suspension. What good is a tight front suspension is the rear is loose and creaky? You can replace diff bushings without dropping the rear subframe and it'll take an afternoon. The book calls for dropping the subframe. If that's the route your shop is taking, a few hundred more in parts should net you a fresh rear suspension.

Sixto
83 300SD