It coudl still be done though, it just means that the engine coudln't be downsized the same way as the sedan hybrids. However, a variable cylinder system could save fuel when not towing, the majority of the time for most vehicles, but at least the power is there when needed. The system could also still use electric assist for bursts of acceleration. Perhaps a hybrid truck could have a battery system designed to work more in "bursts," accelerating using full-out electric assist from stops using a pack of ultracapacitors rather than using a more gradual assist to supplement a low-torque ICE. This would allow the trucks a quick bit of passing power without needing to downshift as often.