I am wondering if it could be the diesel shutoff valve. My SD would only shut off with the shutoff lever under the hood. Replaced the shutoff valve just below the VAV and it solved the problem.

Cannot figure out how to copy the link but it is shown in post #13 of my thread named "1981 300SD will not shut off..."

Maybe it is just a coincedence that it went bad at the same time other problems appeared.