I would take a look at your transmission. You would know if theres a external filter. they are hard to hide and usually would be found on top. Theres a chance you may not have one. The earlier galants had them for sure 1999. You can find the internal filters on rock auto. A Rule of thumb is to put in the same amount of fluid that came out provided that fluid level is in the correct part of the dipstick while running and warmed up. I recommend sticking to mitsu auto fluid. Autozone sells a shudder additive for the transmission fluid, I used it on my 03 outlander. Maybe something you can test before changing the fluids.
Shudders at highway speeds could be suspension or wheel issues. I have never heard about a auto doing it at those speeds but I could be wrong. good luck im not on usually.
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