Previous application of rear axle has close to nothing to do with being 4 wheel drive or not when used in a different truck.
If yours was from a 4 or a 2 wheel truck it makes no difference. It is the same work with a newer rear as it would have been with the original to make your truck 4 wheel drive.

In my opinion it's not worth converting "just because" but if you genuinely have a reason and desire then go for it.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-