My 93 Dodge W350 will down shift as I am driving as soon as I let off the fuel and it drops speed about 3-4 miles slower than I was going. It will do this at any speed over 35 miles an hour, even at highway speeds. I was told that there is supposed to be a spring on the linkage cable for the passing gear where it goes into the transmission and if this spring is missing this will happen. I do not see where a spring hooks to or connects at this point. Any thoughts?

Check for codes. I suspect the Vehicle/Transmission Speed Sensor.