What he said.
If its of any help, I am going to be changing both my gearbox and my transfer box in the next week with a bit of luck, ive got a reverse fault with my main box and im taking the opportunity to put a higher geared disco transfer box on. If it turns out that you want my transfer box, then you are welcome to it - if you don't mind i will have to ask for what my disco one cost me, but this shouldn't be more than £50 tops.
I have heard of mainshaft wear, but as above the R380 'box (reverse right and down, 'under' 5th) this is much rarer.
I am not a gearbox pro, and don't claim to be really, all I have is a working knowledge of it.
The transfer boxes are not very complicated though, they have no synchro to worry about, and only two speeds, so they are very simple internally, and all the spares are about.
From what I have gathered also, swapping the entire unit is pretty easy, ask me in a few days an ill tell you!
Let me know if you want/need my x-fer box.
One other thing you can do - put a powerful magnet on the drain plug of each gearbox and run the engine in gear for a minute or two, then switch off and drain the oil. The magnet will attract all the steel filings to the drain plug, and you will be able to see which box has taken the most beating by the metal soup. It does however sound transfer related if it 'speeds up' with higher gears.