The problem with a differential change is that he has a 4wd so it would require a double differential change otherwise the 4wd won't work. I'm not aware of an overdrive tranny that fits the 617. What about some kind of aftermarket splitter that would allow overdrive in 2wd but keep the original ratio for 4wd. Then you could continue to use the standard 617 diesel trannies.

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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