The wheel sensors can trigger the light.
The " parking" brake light will illuminate when there is a brake system problem believe it or not.
It can also indicate low fluid in the reservoir too.
When you turn the key on the system does a diagnostic check of the circuit. When it detects a fault the abs light in the dash will illuminate.
I'm not sure exactly how it knows if the rear wheels lock.I've been told it knows this using the speed sensor located in the transmission. (Bolted to the side of it)
on the 2 wheel drive trucks the front sensors are easy to remove but since you have the 4X4 you would have to buy the hub since the sensors are built into it. And they are $$$.
The sensors can be tested with a multi meter at the plug. Search the net. That's where I found this out. I tested mine and they were both dead.
I sprayed brake cleaner on mine and they both quit working.
Both of my lights were on on mine. I got mine from a junk yard for less than $30.
The connectors are tough to access though.