With a 24 inch extension the manufacturers reduce the load rating of the hitch by 50%. If your truck has one rated for 500 lbs. then you can safely add a 250 lb. load with the weight of the rack added into the calculation.
My truck's factory hitch is rated for a 1500 lb. tongue weight so even with an extension I could put a 100 lb. rack with a 650 lb. load using a 24" extension. I would not dream of adding anything close to this much weight at the rear of the truck.
For my part I would rather put the weight on the front axle and use a front mounted receiver to do so. The downside is needing to have a custom rack that will be low enough not to block the radiator and impair the engine's cooling system - but it has been done.