the engine applies very little drag, since it has no throttle plate, so there is almost nothing to be gained by downshifting other than clutch wear. the brakes on the other hand are very substancial for this sized truck and will stop you just fine. downshifting from 5 to 4 and 4 to 3 should not require doudle clutching as the syncros are very robust and the rpm change is small, going into 2 from 3 it may help a little in situations where you are then going to accelerate without stopping. you should never attempt to shift into 1st or rev unless at a complete stop as this can cause serious trans damage.