Why would your 1994 Nissan truck V6 jump out of overdrive and downshift as soon as you let up on the gas at speeds over 40 mph?

Overdrive is designed and set to only work when you are at full throttle. The overdrive switch is located at the end of the accelerators travel on your carb or Fuel injection. You can adjust it to work at a lower level,but I wouldn't recommend it as it is only for emergency acceleration and hills.