What is happening when your 99 dodge durango shifts out of over-drive on the highway and trans temp comes Is transmission going out?

Check transmission fluid: If the transmission fluid is low, it can cause the transmission to overheat and shift out of overdrive. Check the transmission fluid level and add more if necessary.

Inspect the transmission filter: A dirty or clogged transmission filter can restrict fluid flow and cause the transmission to overheat. Inspect the transmission filter and replace it if it is dirty.

Replace the transmission solenoids: Faulty transmission solenoids can cause the transmission to shift erratically and out of overdrive. Replace the transmission solenoids if they are not working properly.

Repair or replace the torque converter: A damaged torque converter can cause the transmission to overheat and shift out of overdrive. Inspect the torque converter and repair or replace it if it is damaged.

Rebuild the transmission: If all of the above steps have been taken and the transmission is still shifting out of overdrive, it may be necessary to rebuild the transmission.