1. When was the rear end oil and filter change?
2. What oil and filter was used in the rear end?
Another area to check is the speed control linkage. It could be out of adjustment. Page 2.43 and 2.44 of a chassis service manual (
https://s3.us-east-2.amazonaws.com/c...ervice+Man.pdf ) shows how to adjust the speed control linkage.
Manual was found in OCC tech section.