I'm currently having an issue with our 2008 Explorer, 4.0L, 4x4 RSC. The RSC light will come on, and there is a dinging sound. This has happened in the past, usually in town, slower speeds, etc. I had it checked out, but no codes were being thrown. The brakes (rotors & pads) are fairly new and in good working order.
Well, tonight it happened at ~70mph. It felt like each brake was individually depressing, even though the brake pedal had not been touched. I assume it's a sensor but plan to get it into the detailer on Thursday.
You mentioned that previously it gave an indication but no codes were being thrown. Were you using a professional scan tool to check for codes or the typical consumer grade that you can buy or borrow from the local parts store? The consumer grade tools typically cannot access ABS or traction control codes.
It sounds like you probably have a wheel speed sensor or tone ring issue. A professional scan tool should help diagnose which sensor is reporting an issue and make pretty quick work of getting the issue resolved.