I just had this same problem with my SD this past Labor Day weekend. I rolled the driver's side rear window down, and it wouldn't come back up... tried both switches to no avail. Pulled the door panel and the plastic "window regulator slider jaw" had broken. Was able to "wedge" a matchbook between the window and the bottom weather strip until I could get it fixed. It was a $7 part.
Robert W. Roe
1984 300SD 165,505 miles