I think it's a common thing with the T100. The 3.4 works much better in the smaller, lighter Taco than the T. I just bought my 3rd T from Wisconsin a few months ago, and drove it straight to TX. Even with the small 235/75r/15 tires at the time, it would down shift often.

Several years ago on my first T100 where I was running 35" tires and stock gearing, the problem was even more pronounce. It got to the point that the temp light would light up often if I was to travel out of town for longer freeway speed.

On my current T, my plan is to just keep tires @ 33", and add a much larger transmission cooler for the truck. By turning the OD button off would also help control the heat, but sacrifice a little on MPG.