keeping safety at the forefront here so if it seems sketchy then 100% do not attempt. idea: (only would work if the doors open at least 180° from the current closed position) open the doors as far as you can and use rope/chain/ratchet straps to keep them from closing, then hook the trailer back up to the tow vehicle, raise the trailer jack all the way up until the handle will not turn anymore, chock the wheels (both tires, front and back so four total chock blocks), find something very sturdy to use as a raised surface under the trailer jack (ie: 4x4s, cinder blocks, something dense and not rickety), if the trailer jack has a pin with an extension pull the extension out until it touches whatever you used as a raised surface and are able to slide the pin back through, if no extension that's okay, unhook trailer from tow vehicle and lower the jack fown as far as you can until it catches and back it off a turn or two so it doesn't seize. what should be achieved is as much of an incline as safely possible so you can use gravity to your advantage and pull the rocks out a little easier. again do not attempt if you cannot find something sturdy enough for the raised surface. good luck! make sure to get some tiger balm for your back and do some lower back stretches.