2000 Isuzu Trooper not going into overdrive

We were in a flood last summer and while leaving our home we had to drive through a few feet of tidewater. A few months ago, the truck stopped going into overdrive. At around 70 mph the check trans light comes on. I've been changing the transmission fluid as much as I can at a time, draining the trans. Pan and refilling with trans fluid, (about 2qts at a time.) This seems to help smooth the shifting between the 1st few gears, but it still won't go into overdrive.