I just bought a 1996 Ram 3500 Cummins. Truck has just over 100,000. The original window sticker says the truck has the HD 4speed option but the sticker on the hood says std so not sure about that. The truck goes in forward and reverse smooth and shifts smooth. The problem I noticed is that right around 55 to 60mph (which also happens to be regulated commercial truck speed) the truck seems to hunt or slip in and out of what I think is OD with about a 300rpm change. I turned od off to see if it would stop in it drops about 700rpm but is steady but if you continue to slow down to say 30 or 40mph with the OD off it hunts and slips the same so that is why I don't know if its actually OD or not. But like I said at around 55 60mph with OD on it feels like the OD is slipping in and out. Any speed above that it doesn't seem to have a problem.
I checked the fluid level which was good but fluid smelt little burnt so I changed it. That leads into a whole world that I have never experienced. I have always had Fords and this is my first Dodge so I don't know anything about the fluids. The dipstick says ATF 3 or Dexron II. Everybody I talk to says never to use Dexron. The manual says ATF 3 for 2000 and older and 4 for newer. So I went to Napa and they told me to use the Valvoline ATF 4. Right on the bottle it says it replaces ATF 3, so thats what I bought and a filter. So I really don't know if that is the right fluid or not. Some websites say that ATF 4 in older trans causes seal leaks but is better fluid. I changed the filter and fluid (most fluid, just pulled pan) and did the recommended band adjustment which was quite interesting. They put that nice square adjuster on there so I had to make a tool to fit so I could use my torque wrench. I followed the service manual and did the 72 inch pounds then back off 2 7/8 turns for forward and 2 turns for reverse. So checked level and took truck for drive and it did the exact same thing, no change.
I'm sure my next step will have to be take to a shop but was wondering if this sounds familiar to anyone. It would be nice that by some slim chance it was something simple to fix but I know with most trans problems there usually not. Thanks for your time.