difference between 4 and 2 wheel drive frame

[cooperhw]

I have a question for you square body guys. What is the difference between a 4 wheel drive frame and a 2 wheel drive?
Reason I am asking; I have a 81 4wd 3/4ton truck that runs and drives. I keep it up at my cabin property in the mountains. Also ended up buying another 3/4ton truck minus motor and trans because it actually has a better body and it's the same factory color of yellow and white. A few years ago I bought a stripped shortbed 4wd frame with no axles, just fuel tanks connected.
To the point. I would like to build a 2wd pickup out of the short bed frame but don't need another 4wd. what has to be swapped from a 2wd frame to a 4wd frame? Or, would I be better off just shortening the 2 wheel drive frame already under the extra truck I have.
Oh yeah, I also have a square body suburban that I bought for the front and rear 2wd axles.
I have been over in the 67-72 and 60-66 forums and wanted to pick the brains of the guys who have done this type of swap before.
Any help would be appreciated.
My current ride.