Where did my overdrive go? 88 bronco 2 automatic 4x4 Eddie Bauer

88 bronco 2 4x4 fuel injected,!automatic tranny. Not sure what happened but suddenly the overdrive doesn't seem to be functioning and it shifts hard in a way. So, start the truck, if it's cold start put in gear no problem, if it's already warm seems to kind of stutter and almost stall until u tap the gas then it levels out and idles just fine in gear. So now we're on the highway at about 55-60 mph, doesn't seem to be shifting into overdrive, let of the gas it shifts to overdrive but as soon as you touch the gas it seems to shift back down again. I know it goes into overdrive when I let of the gas because 1 the RPMs drop without the feeling of the gear slowing you down, you feel like your coasting. But again you touch the gas it seems to jump right back down and the RPMs go back up. Like a week ago when driving this bronco it worked just fine so something stupid just happened I'm sure

Dropped pan and changed out tranny fluid and filter. Fluid was almost black but after change, refilling fluid and test drive still no overdrive. An ideas anyone?

These 2 plugs were found near the tranny. The round one was plugged into the tranny and I beieve its the neutral safety switch, the other plug I have no idea because it was just hanging there, I can't find a spot for it anywhere plz help