Bad flexdisc wouldn't explain why part of the shaft is turning and part isn't. It sounds is if the spline that joins the two halfs of the driveshaft let go. Never heard of that happening before. As I recall, there's a big nut that holds the spline in position. Perhaps that loosened up.

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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