Transmission?

If you have the four speed overdrive transmission, stick with what came in it originally and forget the synthetic stuff. The computer is real touchy about detecting any slippage in the clutch packs. When it does, it defaults to "limp" mode and stays in second gear. Different types of fluid change the characteristics of how those clutch packs grab and lock up.

Besides the slippage issue, a shudder during torque converter lockup has been a real common complaint when the wrong fluid is used. That occurs in third or fourth gears as low as 35 mph if you're under light throttle. To verify a vibration is being caused by the lockup clutch, hold the gas pedal steady above around 40 mph and momentarily tap the brake pedal. The clutch will unlock and relock a couple of seconds later. If you feel that shudder each time you do that, suspect the wrong fluid is in it.

There's no drain plug on the torque converter. The only way to get all the old fluid out is with a flush.

I only did a few flushes before I left the dealership. At school, a different person taught the Transmissions class, and his opinion was that in high-mileage transmissions, the grit was in the fluid and no longer on the clutch plates, so doing a flush made things worse. Don't know if I agree with that because having junk flow around with the fluid would chew up rubber seals.

My observation is the flush might be okay for maintenance if there are no current problems, but it won't solve a problem that has already developed. Once it defaulted to limp mode, there will be at least one diagnostic fault code stored in the Transmission Computer, (EATX). You'll need to visit a mechanic with a scanner that can access that computer to read the code(s). That's the starting point.