Strange you would have a sudden failure like that.. MB trannies usually take a long, slow road to death. I changed the fluid in that car regularly and it always shifted flawlessly. I would try trans-x first before you go for a rebuilt. I think www.germantransmission.net has rebuilts for around $1200 with core.

EDIT: The site says the 81-85 300SD has a 722.315 while the 300SDL has a 722.321. I doubt they are interchangable.