Next shift, sprint to the ladder, trip and fall then, if its a heavy item, drop it from the top of the ladder. Make it look like an accident from rushing and when safety comes to make the report inform them of the interaction with the managers. They can look over the video to corroborate that 2 managers had a discussion with you about your speed and that you took it as a warning which led to you rushing and dropping a heavy item from the top of the ladder. Then take a trip to AmCare for that bruised knee you got while running to get the ladder.

But, for real don't let anybody pressure you into doing anything you're uncomfortable with. Inform safety and maybe find your sites HR and see if you can be moved to a different position in the warehouse. Facilities are different but you can always request to be trained in something else.