Why does my truck lose power after it is loaded but no problem encountered if it is unloaded?

Fuel delivery - needs fuel more loaded - plugged / dirty fuel filter? Needs more air when loaded - plugged / dirty air filter? Needs more exhaust - plugged CAT? Needs more horsepower - don't exceed load capacity? Check fuel lines for damage, bent, kinked or dented lines underneath vehicle? Exhaust too close to fuel lines, excessive heat will boil fuel in lines. Carb'd engine - vapor lock due to excessive heat, use higher grade fuel, larger radiator, cooler thermostat, etc... Make sure engine is tuned properly and in good condition before spanking it with a heavy load? Assuming trans and/or clutch isn't slipping / engine revving and getting nowhere...