Does your torque converter not stay locked during decel with T/H engaged? Mine does; I'm not the original owner but cannot find any trans control module, and do not have an exhaust brake.
Double tap the T/H button (turn off then back on) to force a downshift from 4th to 3rd. Trans will stay in 3rd locked as long as you don't touch the throttle.
Mine maintains 4th lockup coasting down to 40mph, and 3rd lockup down to 25. Manually select 2nd and you get 2nd lockup as long as your speed is above 25.
That is how my truck acts, I'd like to know if yours does the same. I thought about adding an EB (Pac Brake PRXB) but honestly don't need it for my size trailer.