I would get a used transmission and put it in. If it works well, problem basically solved. If it works so-so, you can live with it, get another or even rebuild using the removed trans or both. I suppose it depends on your tolerance for having the car out of commission, and your interest in doing internal transmission work. For me, I have an old truck I can drive, and having rebuilt an automatic once before (and enjoyed it - a chrysler 4 sp electric shift, from a minivan), I decided to experiment. So I disassembled a spare trans (that had failed previously) and I decided to try the rebuild myself. I'm about 80% done putting the best of 2 together - we'll see if it works.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.