My truck has recently started with an interesting steering issue. After I've been driving around for a while, I've noticed that it becomes harder to turn the tires. I don't really know how to describe it. I notice it mostly when I try to change lanes. I have a little bit of slack in the wheel, and then it becomes pretty hard to turn, and then it's easy again. Also, it seems like it likes to pull the wheel straight again more than usual. Any ideas?