Any truck that has a DT466 installed will have an Allison automatic, and almost all Allisons have a lockup convertor (99% of 466s will not have the AT series which is the only one that doesn't). They do not operate like pickup truck automatics; they lock the convertor solid in the middle of 3rd for the older ones and the middle of 2nd for the more modern ones, and at any time the throttle is at idle in any gear so long as there is forward motion, i.e. right up to the stop sign. The convertor clutch does not slip, period. Transmission oil pressure is held constant by the regulator and unless there is serious leakage, is not variable based on engine speed.
It makes for harsher driving but it is a much better system than pickup truck style.