What you're describing is somewhat normal in our trucks but can be exasperated by several factors, including tire type and caster alignment.
1. Tires. Tires with a large amount of flat surface area (like performance tires) are particularly prone to "tramlining." That basically means your steering has a tendency to follow the contours of the road. This is especially noticeable in the city, when you stop at a light and there's pavement rutting (see picture below). Tires with an all-terrain profile tend to be slightly less prone to this.
2. Alignment. There are three primary adjustments for alignments: camber, caster and toe. Most alignment places just check your "toe" and "camber." Your truck's "caster" alignment is often the "forgotten adjustment" and affects the steering's tendency to return to – and stay – straight ahead. The more "positive" caster you have, the more stable your truck is at high speeds and the more effort is required to steer away from center. The less caster you have, the easier it is to steer from center and the more susceptible your steering becomes to outside influences.
Caster is usually adjusted with shims and it needs to be done by someone who understands alignments and the caster specs of your truck. This means you'll want to avoid places like Firestone and Sears. I would probably go to the dealership for this. Another great place for alignments, believe it or not, is a high-end sports car tuner. The best alignment I've ever gotten was at a shop that specializes in tuning German race cars. They often tune cars based on feel and those guys know their stuff.