I'm no 4x4 expert - others here are much more qualified to answer your question than myself - but I believe that the axles themselves WILL bolt into your '78.
My understanding is that they aren't as robust an axle (the newer ones are built lighter) and the driveshafts may not be a bolt-up switch, depending on the transfer cases used in the 2 different trucks.
The axles themselvs will swap right in.
The only thing is you might want to look into the u-joints.
The front should be a direct swap.
The rear u-joints on an 87 I believe are 1330 series and on a 78 they are 1310.
But you should be able to get a hybrid u-joint to work.
I know they make them for a 14FF swap.
I might be wrong on the u-joint s in the rear though. I am not sure when they started using 1330 series u-joints.
The axles you plan to swap will infact bolt right up, I swapped out the axles from a 74 1/2 ton to a 78 3/4 ton and they bolted right up in about 6 hours of labor. * Becareful of the brake lines and the proportioning valves when doing swaps. If you swap out axles I recomend upgrading to a 3/4 ton or even 1 ton to strenghthen the rear end.
The 3/4 ton axles are a dime a dozen and you could probley pick one up for around $250 bucks.