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The general consensus is "a lot." But I would imagine specific costs would depend on what truck you have, whether you use new or used parts, and who does the work. I've seen some posts lately that have estimated thousands.
Some trucks/SUVs are a lot easier to swap over to 4wd than others.
It really depends what you want to swap over to.
We had a post in here last week about this very subject. Most SUVs ride on 4wd frames already, so a swap is relatively little sweat. The 73-87 GM full-sizers and 80-96 Fords are semi-easier, and on and on....
for an 81? find a 4X4 in the same year range that got T-boned to hell or something in a junk yard. Front axle, rear axle, transfer case, possibly tranny, and drive shafts. Anythig from a junk yard will need new brakes, lines, fluid and what not. Depends on what kind of trucks you can find in a junk yard. You could find a donor truck in a junk yard for a grand or so, depends on how much they wanna screw you over actually. If you need a tranny it is garounteed in the 1000's. What you could do is search auto trader or truck trader and find a 4X4 f250 with a load of miles for cheap, and just swap over the parts.