It's hard to really say. Are you saying there's an obvious problem, or they just don't feel like they used to?
Assuming you live up north, I would highly suggest you crawl up under the truck and visually inspect the hard metal line, I have seen and worked on northern cars with the brake lines completely rusted through due to the salt etc that cars are put through in the northern winters.
Now, if that checks out, things to look at would be the front rubber hoses, they can be old and expanding too much resulting in soft pedal and less stopping power. You may also have a caliper that just isn't performing as it once did. Your rotors and pads could be glazed over, if you have rear disc that could be the case out back too. Or if you have drums, they could also need replacing, along with the cylinders in the back. Hell, you could even a worn out master cylinder.