If you have the room to drag the rack out of the trench, then just lever the rock one end at a time and slide in smaller rocks or cut logs about 6" - 8" in diameter and about 24" in length. lever the rock up out of its hole one end at a time. Then place some boards down in the trench and slide in some cut fenceposts using the levers. Tow it out by placing the rollers in front, if it drops then just lever it up and place the roller back under. I have dug up some pretty big rocks in my yard by this method. This was before I purchased my tractor. But one of the rocks was 5' - 6' in length by 4' in width and was 3.5' to 3' in height.
Dave