The cooling fan does pull the heat from the trans cooler through the radiator and may not be letting the coolant temperature drop enough as is goes through the radiator. It would be best to hook the trans cooler lines back into the original trans cooler in the radiator, unless of course the internal cooler is leaking allowing coolant into the trans and/or trans fluid into the coolant. Are the trans cooler lines in good condition where they attach the radiator? Is there enough hose running to the external cooler so you can leave it hooked up and move it from in front of the radiator to see if the temperature stays within normal range with the truck in gear? Only do it long enough for the test so the transmission doesn't get too hot. The "coolant bath" cooler inside the radiator is more efficient than an air only trans cooler. For a 92 truck it may be best to replace the radiator and ditch the external trans cooler, and as giffo recommended, make sure the fan is pulling plenty of air with the engine hot. You should be able to hear it and it should suck a piece of paper to the outside of the radiator with no problem. Remove the upper core support and check between the radiator and condenser for debris and rinse the fins with a hose.