If an outside dual is flat, a practical substitute for a jack is to run the inside dual up on a small rock or log. This lifts the outside tire off the ground so you can change the tire.
Another simple method, good for any wheel on an allwheel drive vehicle, is to rig an inclined plane with two logs of suitable size (or a stone and a log). When the vehicle is driven ahead the axle is pushed up the log until the wheel clears the ground. Set brakes and block the vehicle securely. You can easily back off when your job is completed.
To raise the front end, make your own bumper jack by fastening a 5-foot piece of timber at an angle to the front bumper with a chain or rope. Move your vehicle backward until the timber is vertical and the wheel clears the ground. Set brakes and block the vehicle securely before working on the tire. When through, pull forward and unhitch.