The most economical way would be to find a donor (wreck or junk) vehicle and start swapping parts. Unless your truck is in extremely good condition though, it's probably not worth the effort. You can probably sell your truck and then add the money you'd spend making the conversion and be able to buy a decent 4x4 ready to go. I did a frame off restoration some years ago, spent $6000 in parts and untold hours in labor and when a guy ran in to me and totaled it got $1800 from the insurance company. Never again.