It's not as much about which gear, but engine rpm. Most newer trucks have a tachometer from the factory. If you look up the specifics of you engine, it will show peak torque rpm. That is the rpm you want to use if possible climbing hills, whatever gear that takes. Also, if you do not have a tach., temperature gauge will show an increase in temperature if you are lugging (too low of rpm) the motor on a long incline.
Transmissions only heat up for a couple of reasons, if you really want to protect it, have a shop install a separate transmission cooler and a temperature gauge. Heat is a transmission killer..