95 12v Cummins. As soon as I let off the throttle the truck shifts into third and shortly after downshifts to second. It acts like it’s in tow haul mode, but it obviously doesn’t have it. Could it be the TPS??

 

It should have tow/haul mode.... Its the O/D off button on the dash. If the PCM is disabling O/D for any reason, the light on the switch should come on.

The first three gears in your trans are purely hydraulic control. If it isn't shifting right, make sure the TV cable is properly adjusted, and moves freely. Otherwise, it's going to be an internal problem.... When is the last time the fluid/filter was changed? Bands adjusted??

 

It upshifts just fine and when it downshifts I just re apply the throttle and it goes back to normal. It only downshifts when I let off the throttle

 

How much of an rpm drop?

 

I don’t understand what you are asking?

 

If you drive in a straight line, at a steady speed, (fast enough that O/D is actually engaged.) and you just move the pedal a bit, does the tach bounce? If so, how much?

 

The tach doesn’t really bounce. if I let my foot off just enough it will downshift into third.

 

It shouldn't downshift when you let off the gas, unless you slow dramatically. Check TV cable for proper operation/adjustment. If that doesn't help, time to visit a trans shop, and have them check pressures.