Clunking sound on highway...
 

While driving my 82 300TD on the highway today, a rythmic clunking sound began from the middle or back of the car (hard to tell). When I slowed down, it became more noticable, but stopped around 10mph. Upon start-up, the clunking was gone, but resumed 15 minutes later.

My rear CV joint boot is leaking oil. Would a CV joint low on grease cause a noise like that?

Dry CV joints usually make a "clicking" noise but obviously that is very subjective. Yes, a CV joint that is dry because the oil or grease has spun out through a cracked boot will make noise. You can drive it for some time in that condition while you read the zillion posts on the subject in this forum and decide how you want to proceed. My personal preference is to replace the axle(s) with CVJ-Denver rebuilt units but the next person to post will have a different take on the situation. That's one of the great things about this forum -- lots of opinions to choose from.