Have you checked the alignment? It it wants to go past center from a turn, I'm almost wondering if something shifted on you. But the comment above about changing the damper would be the first place to start. Without knowing the mileage on the truck, I'd bet that would tighten things up drastically. if it finally lost it's charge of oil/gas, it wouldn't provide any damping to steering movement.
they're what, 20 bucks? Save the pump till later. Try the cheap stuff first.