The first thing I'd do is stop replacing parts and start gathering information.
The fact that the transmission shifts perfectly under heavy load going uphill is an important clue. If it only acts differently while descending with the engine brake engaged, the problem may not be the transmission itself.
Modern transmissions make very different decisions during deceleration than they do during acceleration.
First, Is It Normal Grade Braking?
Many trucks are programmed to hold lower gears while descending hills.
Depending on:
Vehicle speed
Brake pedal input
Engine brake operation
Throttle position
Transmission temperature
the transmission may delay upshifts or downshift to increase engine braking.
Some gear changes that feel unusual are actually normal operation.
Verify the Complaint With Live Data
Instead of replacing more sensors, we monitor:
Commanded gear
Actual gear
Input speed sensor
Output speed sensor
Torque converter clutch status
Transmission temperature
Throttle position
Brake switch input
Engine brake request
Line pressure (when available)
If the computer is commanding the gear changes, that's a very different diagnosis than if the transmission is failing to follow those commands.
Check for Software Updates
Manufacturers occasionally release updated transmission calibrations to improve shift quality, grade braking, or gear hunting under specific driving conditions.
A calibration update can sometimes resolve a concern without replacing any hardware.
Evaluate the Engine Brake System
If the engine brake isn't functioning correctly or is sending inconsistent signals, the transmission may continuously change its strategy while trying to provide engine braking.
The transmission and engine controller work together during downhill operation.
Inspect the Valve Body and Hydraulic System
If scan data shows the transmission is being commanded correctly but isn't responding consistently, we begin looking at:
Pressure control.
Valve body operation.
Solenoid performance.
Hydraulic integrity.
Only after confirming the electronics are working properly do we suspect an internal hydraulic problem.