Delayed 2-3 shift on 1996 Dodge Indy Ram

We have a problem in our truck where you have to let off the gas in order for it to shift into 3rd. This does not happen all the time (about 30% of the time). It has about 70,000 miles on it and is well maintained.
Oil has never been changed (and I won't according to most good mechanics and an Engineer I knew at GM Testing Track - who could not explain [GM that is] why the Bonnevilles acted this way)

Can anyone offer solutions (that are cheap... lol).

thks all . Ken

vERY COMMON PROB.,SMALL PARTS NEED CHANGING IN TRANS,TO HELP PREV THAT FROM HAPING AG, RUN A EXTRA TRANS COOLER AND RUN TERMOSTAT AT 170/180. HEAT IS CAUSING PARTS TO FAIL. YOUR PROBOBLY AT 195/200PLUS AT TIMES