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I don't think they are stock,I put a set on my 86 f-150 as the ride was better then when I was running air shocks,at the time they were cheaper then a good set of gas charged units.
Boosting the load capacity with shock absorbers is done all the time, but not by the factory. It's bad design practice to put all that load continually on the small pins where the shock mounts.
If you look at the first picture on the top, you can see the leaf spring design, and how the smaller arched leaves on top are above the larger flatter leaf on the bottom. This gives the truck a nice soft ride, and you will get this back if you take those shocks off. When you load the truck down, the upper leaves will come down and hit that lower leaf, and then stop for the most part, so you can carry a heavy load. This design makes the truck squat a little bit loaded, which bothers some people, but won't hurt a thing.