Suspension help

[ramo]

Greetings everyone,

I am the proud new owner of an 84 C-30 Sylverado 1ton 2-wd. I have owned the truck almost 6 months and have gutted the cab,replaced hvac system,shocks,gauges,dash,carpet,all knobs and buttons.
I recently had the oil ,tranny and diff gasgets replaced as well as all fluids changed along with brakes and axle seals.

My problem is that at speed when i hit a bump or hole the truck just goes left or right and at highway speed 55-65, the truck is a fight to keep straight because it drifts something terrible.
If anyone can shed some light on this problem i would greatly appreciate it as i am at a loss.