Belvidere9824valve

Hey, guys! I'm 17 years old and getting ready to start driving a truck to local farmers markets and have a bit of a problem. Me and my dad are the proud owners of a 98 Dodge Ram 3500 refrigerated box truck. It has the Cummins 24valve and an auto tranny. The problem is that the transmission doesn't shift into overdrive properly. It has several lights that come on in the dash like the trans temp light, the airbag light, the o/d off light and some others that I cant remember. Funny thing is, sometimes when we're sitting at a stoplight ect, all the lights in the dash go out, and the trans shifts fine! The company that had the truck before us put another used tranny in it, thinking the trans was the problem. It wasn't. Could it be the computer, or grounds? My dad had the computer go out in his 99 ram 2500 360 gas and had some of the same symptoms. Also, what rpm can I safely run this thing without hurting it? We're not really worried about fuel mileage right now, as we're only gonna drive short distances till we get the overdrive fixed. It has 3.55 gears. I only know the basics about diesels, but I'm trying to learn.