90 Trooper 2.6L A/T over drive Problems

I have a 90 Trooper 2.6L A/T that has a suddenly developed a problem with the overdrive. I was traveling down the road when suddenly I lost overdrive and the transmission started slipping. The only way I was able to get home was to accelerate hard and then let up and coast for awhile, then accelerate hard and coast. I had to do repeat this process until I got home. I also noticed that the O/V light blinking on the middle console where the shift lever is. Anyone have this problem before? Is it a sensor causing this problem?

I have an 88 that won't go into overdrive but no lights come on. I just got the truck and I'm actually not even positive whether it's not getting into 4th gear or overdrive, but a few times the RPM's have dropped lower on the freeway and then gone back up to stay.

So have you found anything out?

Thanks.