So you think the springs are bad? You started the post by saying "All is original". Even if the springs are weak the pinion angle should not have changed drastically. You even have talked about referencing other vehicles for the angle. I can understand that if the springs are junk you could have these issues. Seems you could have easily determined there was an issue long ago just looking at the springs and inner liners, are they still there? Mine still has the "Original rebuilt springs" and never had pinion angle issues before or after rebuild. Hope your not throughing more good money for bad at it.
