Sounds like rear end to me. Pull the cover & have a look. If the pinion brg is tight & the carrier brgs sre loose you can adjust them up & help it some. It is easy to change the center section if you find a good used one.
Those old aluminum Grummans are great trucks. That's practically all I worked on back when I had a shop.
Have you posted a photo in the Gallery?


They say money can't buy happiness. It can buy old Chevy trucks though. Same thing.
1972 Chevy c10 Cheyenne Super
In the Gallery Forum