looking to change my axle ratio

Yes two identical trucks one with OD one without the one with OD will have a slower engine speed in the last hole.

 

In last gear? Higher engine RPM? Your question confuses me.

 

I think you're confused. Which part of the transmission, the front or rear? The rear would be spinning the same as the other truck, the front is faster because of no OD gear.

 

Yup that's what I thought, you're confused

You don't run a direct tranny with the same gears. Our theoretical trucks, the direct transmission one is spinning at 2055 rpm and the overdrive at 1500 rpm. Both have 3.55 gears.

Now put in 2.64s in the direct transmission equipped to get the engine rpm back down to 1500, what happens to the back half of the transmission and the driveshaft?

It slows down for the same road speed.

 

The transmission spins at engine rpm in the front part, driveshaft rpm in the back half (ignoring the range selector). He's right in that if a transmission runs hotter in direct there's probably something wrong with it, OR it's because the driver is using more power (pulling a hill for example) than the overdrive gear.