Fleetwood/Roadmaster/Caprice/Olds wagon all use the same intermediate shaft. The boot on the upper joint changed over time but they're all the same. The intermediate shaft has two things that wear out--the rag joint at the bottom and the joint at the top. The shaft is out of production, and some people have bought aftermarket ones or rigged a Jeep one to fit.
I bought one on Ebay a couple of weeks ago. It comes compressed, so you have to pull the two ends to extend it, but it's the right NOS part. Put it in and it made a nice difference.
The seller appears to have a few more for sale. Search for "7831211" in the description and they'll come up.