Hard Shifting

I have a 2001 Chevy Venture Van and its been shifting hard for 4 months now,,My uncle in Tenn. said since it shifts good while cool,,then harder as the heat builds it was more than likely my High Shift Sensor,,,although he thought it went in a plate inside the fluild pan,,,well it doesnt,,lol,,does anyone know where it goes???,,it has a cylinder shaft on one end with 2 "O" rings an a plug for a double wire on the other..any help would be appriciated.

What transmission?
What code no is it setting?
MOST SOLENOIDS ARE ON THE VALVE BODY INSIDE THE SIDE PAN.
MT

I have a 2000 Venture with identical problems. The tranny shifts fine for trips around town, but shifts hard on longer trips, when the transmission gets hot. I have to pull over and let it cool, then it's good to go, again. It also whines during the time of hard shifting. The transmission is original from Chevy, but I don't know what model it is. Any assistance on this one is appreciated. Thanks.