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I will try to be brief. Here is the situation, 143K miles on truck, 1994 F250 5.8L 4X4. It has happened about 4 times, the transmission will just shift into 2nd gear while travelling at highway speeds (70 mph) the truck doesn't try to gain speed it just slides to a different gear. It has also had an extemely hard shift from time to time, all gears.
The fluid is very clean, good color, no burnt smell, and no leaks.
The Od light has not flashed, and does work properly.
Replace the Transmission Range Sensor (TRS.) It's the sensor on the driver's side of the trans, where the shift cable attaches.
As it starts to fail it does just what you're seeing. It will do it for a long time before it gets bad enough to set a code. Usually the trans is destroyed before a code is set.