I have a 1999 Ford Ranger with 125k miles. The anti-lock brake light stays on. On rainy days the brake on one wheel sometimes grabs, but not always. What should I do?

The ABS system has detected a fault, set the ABS light on, and turned off the ABS system. This should have no effect on the brakes at all, except that the ABS system will not kick in to prevent a wheel from locking up on slick or slippery pavement, as your experiencing. The cure begins with having the ABS system scanned for codes to determine the fault in the ABS system. Most shops that do brake repair should be able to do this. Parts stores, like Autozone or Advanced Auto probably cannot read these codes, since this is different from the OBD-II codes for the engine management system. Once the fault is found, then the repair can be estimated. Sometimes it is a simple as a wheel sensor that has gotten crud build-up on it and cannot read the wheel speed properly.