You have greased those shackle pins, yes?
And just to foot stomp what's been said before .... that truck was not designed for more than 50 mph (as originally built ...) with that in mind, I think it would be imperative for you to completely redo your front suspension and steering. Replace the tie rod ends (if you can find some). You said you will be replacing the king pins -- good! And rebuild the steering box. Also redo your drag link, if there is a kit available to do that. I did mine on my '49 1.5 ton. None of this will likely fix the hop (I'm guessing that the suggestion to put some weight on the truck is spot on ...), but it will keep you alive if you hit a pot hole or a stick or wiggly grooves in the road at 60 mph.
Back to the hop .... Again, to reiterate what's been said above -- that truck was designed for carrying weight around. Empty, it will have a *really* stiff suspension that will not be fun to drive on, especially on bumpy roads. Before getting too crazy, put some weight back there -- at least enough to deflect the springs a little. Make sure your load is secure
and check your tire pressures again. Then go see if the ride improves.
Always check the easy, stupid stuff first. (my paraphrase of Occam's Razor