I have switched truck beds in the past and warn you to be very careful because the box is a giant dead fall waiting to squash what ever is near. Pulling out from a bed will drop the box 4 feet???? to the ground and will sure mess up the box. The box could come off the truck sideways and roll. I have used 6x6 timbers and drums to raise beds enough to pull the truck away slowly. Never get anything in a pinch point, your hand, head or other body parts. I would suggest you use a bank to back up to and lay timbers down for bed to slide backwards onto earth. Another option is to back a rollback truck to your truck and jack up box from frame and slip 2x4s across frame rails and winch box to rollback and then lower roll back and pull bed onto ground with timber blocks underneath. Remember that metal to metal surfaces will slide in a heart beat but a layer of wood between metal slows sliding. 2 inch iron pipe makes good crush free rollers but stuff can slide sideways off the rollers. Two or three 2 inch 24 inch wide pipe rollers between two 2x12 timbers will work wonders and if it falls the load only falls 2 inches. Keep well back when things are moving, and if it looks scarey, stop and do it safely. Better to rent a crane than a casket or months of rehab. Mark