I had the same problem when I first got my truck a couple of years ago. You move the lever to 4WD and it was slow to engage, you might have to drive several hundred yards before the 4WD light came on. I did two things that seemed to fix it, as I've not had any trouble with it since, but I'm not exactly sure which one was the fix as I did them both at the same time. First, I unplugged and replugged the connectors. If it was an electrical contact problem, this may have fixed it. Second, I replaced the front differential fluid with Mobil 1 (I think it's 75W-90). The old fluid looked thicker than what should have been in there, and maybe that was making it engage slow, or maybe the synthetic just works better if you aren't in the habit of using 4WD a lot. It's been working greater ever since.