Got the exact same problem on my 85 300TD. A rhythmic thumping that occurs only with the drivetrain unloaded. I had assumed it was the center bearing but haven't looked or replaced it. I'm glad you posted and that I didn't have the bearing replaced yet since now it doesn't seem to be the likely candidate.
I asked a similar question about it on the diesel discussion a few weeks ago. Someone suggested tires. Perhaps, but I don't understand why it can only be heard with drivetrain unloaded.
It doesn't sound like the rear end in mine.

Perhaps this is an oddity with the TD's.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13