What kind of transmission fluid do you use for a 1998 dodge truck?
my husband has a 1998 dodge truck and he uses GM Dexron-III/Mercon Automatic Transmission Fluid. You can buy at any automotive store. Dodge,Chrysler and Jeep products require ATF + 4 made by Chrysler. While it is okay in a pinch to top off with GM Dexron-III/Mercon, replacing it with that will cause you problems. They are different.
Check your radiator for signs of transmission fluid ( you will know as soon as you open the radiator cap) oily, creamy, gunky buildup. The transmission cooler is built into the radiator core and it is common for this to leak allowing coolant into the transmission and transmission fluid into the cooling system.