I have a 1998 Ram 1500 that on the end of a 1000 mile road trip shifted out of OD while rolling down the road. I hit the OD on/off switch a couple of times but it had no effect. The next day it seems to be working again as it should,but I have only driven around town. The fluid level is normal and doesn't appear overheated. Where should I start to resolve this? Thanks

Look and see if it has any stored fault codes in the PCM.

I had the same issue with my 99' Ram. Turned out it was the OD switch. Once it was replaced it was back to normal operation.

If it has a transmission cooler you should check it and see if the fins a plugged with dirt/bugs. There is a sensor for transmission temperature and if it goes over a certain mark will send the truck into 3rd. Makes sense that after driving for 1000 miles with blocked air flow to the cooler that eventually the temp would creep up.

Just a suggestion.

Hose the fins down until you see clear water running out the bottom.

Hope it works, if anything it will cost you zero.

I had an isse a long time ago as well.. Have to make sure all connections are tight and clean. Also, when I changed my tranny fluid and filter, I made sure all my internal connections were secure and tight. I also shot some CRC electrical contact cleaner and reconnected.