First of all, you don't have an 84' SD, that means 84 feet. Drives me nuts, it's '84 (apostrphe goes where you removed letters/numbers).

Moving on, the '86/'87 SDL should have a similar transmission, with a couple of important differences:

The '86/'87 SDL engine revs to 5150RPM, the '90/'91 SDL engine only to 4650RPM. The early engine also makes its power up higher.

What that will mean is most likely your car has a lower-stall torque converter, which you can easily keep on your car after a flush, but also much earlier shift-points.

Whether the early transmission can be adjusted to shift lower just with the Bowden cable adjustment I don't know, and the downshift solenoid will also try to spoil the party, meaning that the engine might rev-limit before shifting at full-throttle.

You could also keep your valve-body to keep your shift points, but that is an additional risk as it could be the problem, and additional work at which point you're pretty much better off repairing your transmission? I don't know.

Best of luck either way, the '90/'91 SD/SDL transmission is not going to be easy to find.

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Gone to the dark side

- Jeff