Haley, I will admit i have never done anything like what you are talking about. But, i do wonder how often you would need a driven rear axle... If it would be used every couple of years, and you plan too keep the combine for the foreseeable future, then I would think it would be worth the work and effort in doing the conversion.
Do you know if the 4wd works on the other machine? I know you said the machine doesn't run so it will be sort of hard to test, but it would suck to do all that work to discover you need to spend more money on fixing the 4wd or that parts aren't available.
Just my 2 cents, sorry I can't be more help.