I had very same problem with my 85 SD. It would shift late at 3000-3500 rpm and shift harshly 1st to 2nd. Turned out to be bowden cable adjustment had too much tension on it. Even though the plastic adjuster is broke I was able to adjust it after a couple of attemps. It now shifts at about 1800 rpm and very smooth. Very easy adjustment and did not have to replace broken adjuster as I heard it is a pita job that requires replacement of whole cable into trans and requires dropping trans pan.