I have a 92 f150 xlt flare side. I was out for a drive and I noticed on my dash a rear abs light was on. What is the problem?

 

It means there's a problem with the ABS system. First thing to check is the speed sensor in the rear axle.

 

What else should be checked just in case

 

I'm only saying this cause I had my own problem and the result sounds similar. I pulled my ABS fuse to disable it until I fix my problem and the ABS light comes on now on start up which is to be expected. The emergency brake light also comes on now on start every now and then. Check to see if the ABS Pump fuse is blown, most likely a 40 amp fuse if it hasn't changed since my 05.