What were the rpm's when the car was going 50mph and what gear was it in?
From your description it is hard to diagnose what is going one because it's possible that the rpm's were increasing because the car was downshifting.
Loss of power in these vehicles is often attributable to dirty fuel filters. I would change the filters, making sure to fill the large secondary filter with a burnable fluid like diesel, Purge, or ATF before reinstalling.
Sometimes the accelerator linkage can fail on these cars causing lack of power also. Failure of the transmission in the manner you describe is less common.
How many miles on the car? How long have you owned it? How does it shift? Hard or soft?

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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