From my experiences an Automatic transmission having problems on a high mileage car/truck is most likely fubared inside. 99% of the time an oil change and filter will exasperate your problem. Original oil has been in there contaminated with clutch pack material slightly opening passages causes pressure variables. Putting new oil in my experience will just allow the trans to slip more.
Your best bet is to bring to a shop that can do pressure readings. At least that will verify what’s your problem. Clutch packs, converter,valve body can be anyone of the problems.
There are definately manual transmissions for a 1500 series dodge ram. My dad had one a 02 2500 ext.cab 5speed. I’m sure theirs one sitting behind someone’s barn just rotting apart.