Lexus GX 2004-09 :: Truck Not Downshifting Properly For Optimal Performance

I've been using my GX to haul the family about 500 miles round trip between our primary home and our lake house. I've done the trip about 5 times now, the route has many elevation changes (hills) and it keeps becoming more apparent that the GX is not downshifting when I would like it.

1) At first, I was stomping down on the gas pedal, entering the uphill stretches, to force a downshift, most of the time it needs to seek all the way to third before it gains enough torque to maintain fast highway speeds (70-80). However, this stomp method results in gunning the engine and then letting off the pedal, not exactly the smoothest process and hard to stay in the power band around 3rd and 4th gears.

2) I finally figured out how to push the shifter to the left and manually shift between the gears myself. However, I have found the downshift from 5 to 4 rarely happens in a timely manner. I shift it over to 4, the light goes out on D but takes a long time to light up on 4, and it stays in 5th gear. I have to downshift all the way to third to get a response out of it, which is immediate.

3) Once I started manually downshifting to 3rd it made a GIANT different in performance of the vehicle over hills. 3rd and 4th gear have plenty of power to move the vehicle along without stomping on the gas pedal and provides a much smoother ride.

Noticed the poor performance of the transmission shift program. I know the 05+ have slightly more power , but this doesn't strike me as a power issue, just that the truck isn't downshifting for optimal performance.