Hopefully more knowledgable people will post here. It has been a while since I had my diesel too. I had the same thing happen to me. It did not cost me an arm an leg. From what I remember they did not even have to drop the tranny. I recall there being some kind of line/switch that leads to/from the tranny. The tech fixed it in a 1/2 hour. Again, I am no guru on this subject.
Except for the inconvenience, I would not jump to conclusions yet on the price. Could be less than 100 clams to get it done at best??
'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)