Thank you internet!

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.