Please help: have you seen a driveshaft do this before? 1982 300SD

[oparrott]

When the car is hot, and in N or P, the driveshaft rotates back and forth, causing the whole car to shudder. If I look under the car, I can see it rotate 15 degrees forward, then back the other way. It just rotates back and forth, making lots of clunking noises until I put it back in drive and get moving. The car is fine on the highway. I'm at 202k miles. Any ideas? I couldn't find this problem in previous posts.

Other details: Around 1200 rpm there's a scraping noise coming from below the shifter, related or not... The scraping at 1200 rpm is consistent but the clunking is not, like I said, it only does it when the car is hot and idling but sometimes it doesn't do it at all. Doesn't seem to depend on how fast I stop. Also, I have to put the car in N to turn it on. But I'm convinced that all the racket is coming from the drive shaft rotating back and forth. Anyone seen this? Do I need a new transmission or differential or just to turn a bolt? Thanks!