Hey there!
I have an 84 short-bed X-tra cab pickup (2x4) with a very rusted bed (suprise) and am wondering if a short bed off an 86 pickup would be able to be swapped out? Also how hard would it be, assuming it's possible?
Will

 

I would assume that yes it will be a direct swap, but could require some fab work. take measurements and confirm. The work involved would likely be to move some body mounts around, but I'd think as long as it's a bed off another xcab you'll be ok.

IIRC I've seen both bed versions on the 84 trucks (exterior vs interior top rail)

 

Direct swap as long as both are either short or long bed.
I swapped a bed from an 86 turbo to my 84 Xtra cab almost 20 years ago.
Using a tractor to lift it off it took me half an hour