Suspension Help 84 Shorty Silverado!?

[swissarmychainsaw]

Hey there, I'm new to the truck world, and this site. My Gradpa and Dad used to have Chevy and GMC trucks when I was a kid, so I thought it was finally time to do it myself!

I picked up a clean 1984 Silverado short bed, and it needs a few things.
307, R700r4, 115K miles.

On the freeway this thing bounces like a beach ball. Nice big slow oscillations.
I figured it needed shocks, so I went to Meineke and put on "stock replacement" Monroe shocks that were about $35 bucks each.

*NOW* On the freeway it bounces like a beach ball. The new shocks seem to make no difference at all.

I am no suspension wizard, but what did I just miss? I don't mind spending some money to make this right, but I don't want to waste it either.

So...I'm asking for some guidance here.
Thanks,
--Nick