You can get another 2x4 and nail them together. Screws work also, and so does glue but it's a little messier. Hell you could even drill some holes and bolt them together.
All kidding aside, its not a very simple project. If you have a 4x4 truck sitting around it may be reasonable, but you'll need to get a different transmission, and a transfer case. Then you'll need to remove the crossmember the engine mounts to and swap it for a 4x4 one.
If you're going through this trouble I would make sure to upgrade to the Dana 35 front end, as the D28s are pretty weak, unless you don't plan on wheeling it.
I'd probably be better to sell the truck and buy a 4x4, but in the end, it's your call.
