The Rear ABS warning light came on in my '92 F-150 302. What do I do to fix this? Is it a real problem if I just leave it alone?

 

Click my signature link, go to the Brakes & Hubs album, & scan down the page to the RABS diagrams. One of them has the procedure to get codes out, and a list of codes with diagnostic steps for each one.

If you leave it alone, you won't have RABS - you'll have regular old-time brakes, unless the RED brake light comes on.