That makes so much more sense! I'd searched online quite a bit but was never able to find a clear answer on exactly how the transmission cooler was integrated into the radiator. After seeing that pic, it all makes total sense now. I was incorrectly thinking that some section of the radiator fins were dedicated to transmission fluid and the rest to engine coolant. Now I see the transmission cooler is suspended in in the engine coolant and thus, is basically liquid cooled.

That would lead me to think that since the engine coolant is staying at the proper temp, the lower bumper airflow must not affect the transmission temps very much. I'm guessing the transmission is simply producing more heat than what the stock cooler can "shed" to the engine coolant before the trans fluid returns to the transmission. It still seems weird that this whole thing cropped up after the new bumper was installed so maybe that airflow is still a factor in some other way or something else is going on...

Thanks also for the tip about the winch sizing. I guess it makes sense that a full-size footprint would be necessary for the loads of full-size vehicles.