One possible thing that might cause a shifting issue is if the o-ring on the filter tube is damaged, worn, or missing.
Amazon link that shows the o-ring as part of a kit:
https://www.amazon.com/ACDelco-TF231...2935891&sr=8-3
I noticed that some kits don't show the o-ring. It is needed or you won't get good fluid pickup and will have symptoms similar to low fluid levels. Including bad shifting.
#7 on the diagram shows where it goes. Another thing some folks miss is removing the old one if it sticks in the opening when you pull out the tube #14.
I would finish your check of the modulator and related stuff before pulling the pan. Pulling the pan probably means replacing the pan gasket + 4~7 quarts of ATF. No point in doing that until you know the problem isn't in the modulator you already have off.