200r4 adjustment stuck?

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i tried adjusting the transmission in my GN theother days its an 87 with a rebuilt 200r4 in it. the shift points are low and the shifts are soft i was going to try to do an adjustment but when i try to press in the "D" button it wont move at all do i have to undo something first can it rust in place etc? I just dont want to break it i was putting quite a bit of force on it. there is a bit of slack in the cable at idle not much but its not taught