Im fairly certain its the TPS or throttle position sensor which is mounted on the throttle body. I believe mine is going out, my transmission "hangs on" to 3rd and 4th. According to my Scanguage which can read the sensor my range is 8%-89% or so.
Right, it pretty much won't shift unless is back off the throttle slightly. The trans fluid is full and pretty clean. I also talked to a trans shop that also suggested the TPS as a culprit. I performed the same test as the recommended with my Scanguage. I'd have replaced it by nowbut as far as I knowits a dealer only Denso part and pretty expensive.
I really don't know what you'd clean, it's a rheostat which is a variable resistor and I think it's connected to your butterfly valve? If you did clean something it'd be the valve to make sure gunk isn't hindering it's movement. I'll be pulling my throttle body and intake plenum soon for a good cleaning as I approach 200k so I can let y'all know what I find.
There are two vehicle speed sensors mounted on the tailshaft that transmit rotational speed data to the ECM in conjunction with the TPS (throttle position sensor). The ECM computes optimum shift points, then sends electrical on/off impulses to the two shift solenoids and the lock out solenoid in the valve body.
Attached is the solenoid diagnostic which indicates the on/off positions for the two solenoids for any given shift point scenario.