Axles...fronts will interchange up till '80-something (87 IIRC, maybe later for the solid front axle trucks that didn't get the new body style until 92). 3/4 & 1 ton rears will "bolt in" but the axle tubes are bigger and will require wider U-bolts
Rears, the 73+ 1-ton rear axles will interchange into the 67-72 1/2 tons/K5's as the leaf spring pads are the same width apart. 3/4 & 1/2 ton 73+ rears will need the spring pads moved in.
I am in the process of swapping a set of axles from a '77 3/4T 4x4 underneath my '71 K5 Blazer, the front axle was an easy swap with a couple buddies, I just bolted the rear in with a piece of 2x4 on the axle tube so that I could roll it in the barn...too cold to work on it out here, when I pay my hosting bill I'll post up a bunch of pics of the swap progress.