I've been working through similar issues with my 77 D. Have you changed your fluid and filter? If not start with that. These transmissions have a mechanical modulator which can be adjusted to soften or firm up shifts. Its on the rear end of the transmission and on the passenger side. There is a 17mm cap on it. Which you remove then there is a screw head underneath. Screwing the adjusting screw in(clockwise) will firm up shifts, and adjusting it out(counter clockwise) will soften shifts. You can also adjust the length of the shift linkage that attaches to the throttle linkage. It controls the shift points. Shortening the rod will raise the shift point RPM and lengthening it will lower the shift point.

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