From your description it sounds as if the problem is that the transmission fails to upshift. Is that correct? There are a fair number of threads on that topic on the forum. First thing to check would be the kickdown switch. Is it stuck down? The shifts in that transmission are controlled by two things--vacuum controls the harshness/softness of shifts and a rod going from the top of the valve cover down to the right side of the transmission controls the shift points. (search on 'bowden cable for that since most later models had a cable and not a rod). There is also a shift kit available to improve the shifts on these transmissions. It's called a Superior shift kit or spring kit. A search on Superior should bring up info on that.
If, on the other hand, your problem is that the car will not move at all when on the ground, you need to be aware that those early SD's had a problem with the ears shearing off the torque converter.

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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