Will manually down shifting the six speed automatic transmission cause undo and or premature wear of the transmission? The truck has 147000+ miles on it.
With this many miles who knows . But why do you think you need to do this ? If you are trying to save your brakes a transmission is a lot more expensive then a brake service.
Now if you a descending Pikes Peak you should shift to lower gears so your brakes won’t over heat.