My truck won't shift into the next gear when I'm going downhill, until I start going uphill. Sometimes I can push the O/D twice and that will work. What's happening?
If the transmission operates normally in all other driving conditions this sounds like a feature in the transmission program to inhibit upshifts to prevent a runaway condition when going downhill in case the driver is towing or a hauling heavy load. You may want to have the fault codes read out of the transmission regardless to ensure there are no other issues such as a faulty shift solenoid or gear monitoring issues, as these will affect the shift characteristics of the transmission. You may want to enlist the help of a mechanic, such as one from YourMechanic, to inspect the transmission and recommend a course of repair if needed.