Well if it's RWD when you've got it in 4L you can skip the electrical/TOD stuff and concentrate on something mechanical - like the first three items on Ldub's list. Drive shafts don't ordinarily go missing but CV Joints are a weak spot - so you might want to start there. Hopefully that's all it is and not a broken chain or some other internal prob with the transfer case...
Just out of curiosity - when you put it in 4L, did that gear it down? It should still go into low range (about 2.5:1) even if power is not making it to the front.