The ABS and BRAKE light staying on could be the sensor in the rear differential, or the wiring between the sensor, and the ABS solenoid located on the drivers rear frame rail just before the axle, or the ABS solenoid itself.

On several of my 91-93 trucks I have removed the ABS solenoid from the frame, and plumbed the brakes as they would have been on the pre-RWABS systems. Then simply unplug the ABS relay that is behind the glovebox on the firewall.

This ABS delete is a popular modification, and for myself and several others better braking, and a better non-spongy pedal feel is noticed.