I'd think real hard if you really really need 4x4? If the answer is yes then you're in for a lot of work and arguably a less than ideal mismatch of old and new and I'm sure compromises, some not so great.
The simplest way (not actually "simple") to do that is probably put your cab on a whole different chassis with the complete drivetrain you desire.
If after careful consideration you don't really need 4x4 in this truck your life just got a whole lot simpler. You can just drive the truck, if you want to go faster swap out the gears or entire axle for 4.10 ratio or close.
In my experience and from watching others you're more likely to end up with just what you want if you drive the truck now and don't rush into tearing it apart and making big changes. First get to know it as it is, perhaps make some smaller changes as you go. If later you decide to do major changes you should then have a good idea of what they should be based on experience.
