The car started making a horrible moaning sound while down shifting. It was coming from the rear differential and that really worried me. The problem with this car is it's 65% newish and 35% forty seven years old. That 35% needs a lot of attention.
I started poking around on the web. The drive shaft had a lot of play in it, which probably meant the rear differential was shot. Rebuilding it was too big of a job for me. I was getting ready to call the shop when a came across a post on IH8MUD. It said to check the pinion nut and see if it had come loose. Sure enough, I pulled the drive shaft off and the pinion nut was extremely loose. I tightened it up and added a cotter pin and it was good to go.
I think I'll change the differential oil just to give it a little treat.