If truck is on lpg look on the side of the carb there is a step up valve it gets vacuum from the manifold it pulls up the engine speed when a load is put on the engine it might be bad see if it moves when there is a load on the engine when vacuum is low under load it moves to help to keep the engine from stalling. Also it is possible there could be restricters in the fitting to the sideshifters to slow it down so the load won't shift fast that could restrict the flow and puts pressure on the engine and could make it stall also you should use the inching brake and increase the gas when sideshifting a load. Hope L was of help.