All the experienced folks will jump straight to the governor pressure solenoid and sensor kit. Before you do that, fashion yourself a test gage and hook it up to the governor pressure port, jack up the back wheels, and then read the governor pressure at various speeds. It should be 1 psi per mph, so it should read 20 at 20 mph.
What I've found can cause this:
- Governor pressure solenoid screen plugged with debris
- Governor pressure solenoid bad
- Speed sensor bad
- Controls screen plugged (it's in the channel plate)
On my truck, the culprit was the controls screen. I ended up changing the other stuff before I found the screen had split and trashed the valve body. New screen and a thorough cleaning got it back to working.
Put yourself an external filter on it when you go back together. Your transmission will thank you.
The real beotch of this whole thing is that a fluid change will cost you $30-40, so if you go transmission diving and don't get it right the first time, you're out $40 again for fluid. You could go armed with everything, but then you are just throwing money at it unnecessarily. My money is on the $1.69 screen and clean the debris off the solenoid screens.
108,000, side terminal battery, TFODjr, filtran external filter.