Does the light go off occasionally then come back on? Mine does this, I got the codes pulled and it was the EGR system. From what I've read this seems to be a fairly common problem with Rangers and will cause the CEL to go on and off periodically.
Hows your gas mileage? Any hesitation when accelerating? Any specific symptoms you know of that have occurred since the CEL came on? A little more info may help diagnose your problem.