Is it rusted out on the frame rails or up by the spring/shock support at the top? If it's just the frame rails like mine what I did was cut out the section of frame that was bad and weld in some 16 gauge sheet metal(just had to replace the bottom half in a few places). If its rusted top to bottom you can go to a local steel supplier and buy the correct size tubing and weld in new peice.
If you want just a quick patch you can also just box the frame by welding sheet metal around the rusted peices.
Oh yeah! Make sure to keep the frame level on all side so it doesn't twist or sag.
Hope this helps